Some thoughts:
We have a 300 litre tank in a side attic (in a gable-end). It's got two heating coils, one fed from the boiler, and a lower one fed by 8sq m of solar panels on the roof. I can't easily measure the efficiencies, but I can tell you that in the depths of winter, the solar still works, when direct sunlight hits it. It doesn't contribute on cloudy days though. There is an immersion heater, which is pretty much only there for emergencies (if the boiler breaks down). It's not on an economy tarrif.
The tank is right next to the boiler, in the exact centre of the loft space because it's tall - it only just fits in - and supported on a separate sub-floor and three (IIRC) joists spanning the gable, above the original ceiling.
After about ten years, it's getting a bit cruddy (unlike Cornwall, we are in a very hard water area), and probably ought to be replaced - I have a lot of nuisance issues with limescale around the system as a whole, but a water softener is impractical (after extensive research, and even buying one and then not fitting it).
So...
Thing #1: wherever and however you fit it, make sure you have allowed enough room to have another plumber wrestle another tank into the space, should that ever be necessary. In our case, you'd have to remove the door jambs from the connection to the bathroom next door, and it would be VERY tight, but should be do-able. The attic is our airing cupboard, and I've made all the shelving hinged and removable, so there is ample room.
Thing #2: You
could get the volume of storage by using two tanks instead of one big one. I can't think of a technical reason not to, and the reheat time would probably be quite a bit better (bigger heating coil area), but the whole installation would be more expensive - tankage would be quite a lot more, plumbing costs higher, and it would need thoughtful design and set-up. But it would also spread the weight a bit, which might be helpful. I considered it for our place, but in the end the plumbers decided they could get a big tank in - and they did. You'd also have to apply a bit of brainpower to the immersion heater aspect. It's probably the case that just fitting small heaters would do it, although you might be fine with the ones supplied, as those are sized appropriately for the tanks.
Thing #3: The plumbing beomes "interestingly" complex: These tanks do
not run at mains pressure (up to 6 bar), but are controlled to 3 bar by a relatively high-volume pressure reducer. This can be a nuisance if some of the cold taps are fed from mains at high pressure, because of the risk of back-flow through mixer taps and shower mixers. They are supposed to have check valves these days (in the taps), but that's usually more about preventing syphoning from 'grey' water in the bath or the kitchen sink (I have no idea how this might ever happen - water byelaws are based mainly on energetically-written fantasy fiction, as far as I can tell).
Anyway, I have several small pressure reducers fitted in the house (kitchen tap, downstairs shower/cloak room and one bathroom), as I found that the kitchen tap, almost the farthest from the installation, was running oddly cold. Our big pressure reducer, on the inlet to the tank, has an extra, convenient, low pressure outlet, which we've used for the top shower and bath taps. Both run really nicely, because hot and cold are "balanced".
Thing #4: These tanks cannot "explode" unless something is done very wrong at installation. All parts of the system: the hot water you actually use, the heating water circulating, and in our case, the solar circuit too, are each protected by a safety pressure valve. Everything except the solar (which doesn't need it) have their vents going via a "Tundish" to outside the building (the Tundish is there to splash a bit (safely!), and make it obvious that the vent has opened). If anything boiled it might do damage, but there is pretty much zero possibility of the tank exploding.
Thing #5: In our place the tank and the gas "system" boiler are right next to each other in the attic. This works well in that it minimises any heat losses in the primary circuit (boiler to tank). For everything else it has some issues, for example, it's harder on the heating pump, as there's no help from gravity getting the water to circulate. And the boiler is a a fair distance from where we want room heat. So I invested in seriously expensive pipe lagging:
The stuff I bought is called "Isover" and came from a local industrial heating specialist. The yellow stuff inside is fairly dense glassfibre. The smallest internal diameter available is 22mm, so any 15mm pipes float about in it, but I've also used it successfully with 10mm "microbore" for one long heating run, with both flow and return going down the same insulation tube. It has a flap of stcky aluminium tape to join the two sides together when you close it up, and you have extra sticky aluminium tape to seal joins. It comes in 4ft lengths, IIRC, and I cut it with an old breadknife (keep a diamond plate handy!). Corners and pipe clips require care (you have to mitre it, sometimes in 22.5 deg. increments, depending on the bend).
It makes a huge difference to the efficiency, and doesn't degrade over time. But it does make pipe runs physically bigger, obviously (it's about twice the overall diameter of the stuff you can buy in DIY sheds). You also need extra standoffs for your pipe clips (off the wall), as it doesn't compress and won't fit under normal 22mm clips - you just need to use pipe clips that have the extra standoffs as clip on accessories.
I would definitely use something like it for the primary run between your stove and the tank, otherwise accept that you will be wasting a fair bit of heat before it reaches your hot water store.
It is very hard to measure efficiency improvements or supposed reduction in household bills in our case, partly because we've improved the heating bit-by-bit as we could afford it, and partly because it's a complicated issue. I can tell you that it works pretty well, that I don't miss the huge, very wasteful old boilers (there were two together!), which probably heated the back garden as much as the house! And, on sunny days as late as October and as early as late April, we don't need the boiler at all for a hot bath in the evening.
Regarding using a stove for the primary heating, this sholdn't be an issue: people use AGAs and Rayburns designed as CH/DHW boilers. You just need an experienced, completent plumber.
The huge advantage of the pressurised system we have is oodles of hot water when you need it in the morning. I usually have a bath, and the rest of the family showers. It does struggle with bath + shower together, as the bath fills really fast, but could cope with all three showers at the same time. (probably - one is a bit Niagara-ish).
When we moved in 20 years ago, we had header tanks for both DHW and heating. The latter grew a huge raft of mould on the surface of the water: disgusting, scary and horrible when I removed it from the attic. The other tank was badly fitted and meant the hot water never worked properly, and again was a health hazard. I don't miss either of them!
Hope that's useful. It's early; I couldn't sleep (normal!); it's very rambling as a consequence. There might be something useful in there. I hope so.
E.
PS: the solar side is
extremely simple in operation, and has been very reliable over about ten years. Unlike the electrical-generation sort the panels probably don't age, and we are still using the original circulating water! It doesn't need any electrial/control connection to the rest of the heating system, because it's lower down the hot tank than the boiler's coil. Everything just works, auto-magically. I love that aspect of it. And it shuts down in summer (for safety) when the roof water temp reaches 104 deg Centigrade - which is quite a bit hotter than the boiler manages. It'll do that most days in high summer, but pick up again if you use the tank water.
