However you go with this I fully recommend using a Megaflow, hot and cold water are at mains pressure and your attic is dry ie no cold or expansion tanks and the such. Had Megaflow for 12 years and no issues
Invented cylinders won't need heating any more than vented ones. You don't need a feed tank in the attic so there's less plumbing. As suggested if you get a twin coil one you can fit solar to cut bills. Solar ones tend to be bigger 300L compared to 200L for non solar. £2000 seems a high quote to replace a tank.
Same as Nigel - solar panels with any excess diverted to immersion heater. Free hot water for about 8 months of the year. If we get several very cloudy days then we use the oil boiler for a boost. I think it is necesary to be careful the hot water cylinder does not stay just warm for an extended time (don't know how long) because bacteria thrive in warm conditions - legionella etc.