My workshop is a kit-bought log cabin and it's a great option for the speed and cost in my opinion. I've now built two, one for my mum (which she works as a therapist from) and my own, which was built as a music room, but now has all my tools etc in. Some things to consider;
- Roof overhang. It makes a huge difference to the weather protection and therefore to how often you need to varnish it. Also, natural light is great, but having sun pouring through the windows is not an advantage in really good weather as you end up either having to put blinds up or have air-con on. I would opt for at least a metre all round (not at home now or I'd measure). The back only has a small overhang and this has weathered completely differently to the front and sides. My Mum's doesn't have much of an overhand anywhere and has weathered badly (rot etc).
- Power supply. Get the thickest cable in that you can. I've had to upgrade mine and it was a PItA.
- Orientation North/South and locating of the doors and windows. Again, to make use of/avoid the light. I've heard Chris Schwarz talking about his favourite area to work being in front of a north facing window because the natural light never glares, so he can work in a consistent environment.
- Timber thickness. Mine's 44mm and I wouldn't go any less than this.
- Heating. I've got insulation floor and ceiling but it gets too cold to work in without heating for about 3-4 months a year (I haven't needed it yet this year for example). That said, it warms up in about 20mins with a 3kw heater and stays warm easily. If I was building again, I would factor in a small wood-burning stove.
I don't know where in North Wales you are, but I'm only in Runcorn (North Cheshire) and if it is practical for you, you would be welcome to come and a have brew and a look at the cabin.