I have a Dunster House log cabin which was built 5 years ago. Excellent design and construction. Erection (or as Dunster House coyly refer to it - installation) took two blokes 3 days, though they would probably have managed it in two but for torrential rain on day 2. I think it would have taken me and a mate considerably longer! I opted for 45mm walls and extra floor and roof insulation. It has served extremely well so far once a couple of teething troubles were sorted out by Dunster House - a faulty lock and a poorly manufactured window frame. The customer service was not great - it took multiple emails plus photos to persuade them to send a bloke out with replacement lock and window unit, but several weeks later it happened and all was fixed in an hour or so free of charge. I think the problem was that the person receiving my emails knew nothing about the construction of their products. Once he actually passed the problems on to someone else, all was sorted quickly.
Yes, it does need some minimal heat to keep frost free in winter - I use a low-wattage tubular greenhouse heater, plus a 2kw convector switched on 1/4 hour before using the shed in very cold weather. During the summer it can get extremely warm -if the sun shines directly on the larger roof slope for too long. Keeping the windows and doors wide open helps, but 30-35 is not unusual when the outside temp is 25. Two fans make this bearable, but do little to bring the temperature down.
My railway runs along two sides of the shed, mostly away from the windows (which I shade during sunny weather) and has not suffered unduly from the temp conditions. The only problems so far have been - i) one turnout (Peco) suffered a buckled rail (where the section is thinner) last summer where I hadn't left enough gap at the joints either side - easily fixed by sliding the fishplate out of the way and sawing 2mm from the rail end. ii) My sector plate fiddle yard requires occasional attention to rail heights either side of the join.
Once the shed was up I made reasonable progress on the layout for a while, but during the last couple of years as a result of family illness I have hardly had any shed time. When things ease off a bit maybe I'll do something of a retrospective build thread about my shed and railway. meanwhile here's a pic of the shed not long after, er, installation and after I'd begun "landscaping" its surroundings.
Keith