A few years ago Jackson's Miniatures introduced a range of affordable British outline building kits because there seemed to be a gap in the market. Since then a greater range of buildings has become available, including some in concrete. Quite a few members of this forum have bought our building kits, and some, like Ian Sharples, have adapted them into prize winning designs. The basic buildings use moisture proof mdf which, when treated properly, is quite suitable for outdoor use. We also do plans for buildings and sell the doors and windows separately. The doors are a little small to scale, but by being clever they can appear to be the correct (scale) height. Quite a few scratch builders use our doors and windows, and also the textured plastic sheets we sell, to produce some very attractive buildings. The benefit of using our doors, and especially our windows, rather than making your own, is that they will last, and the glazing bars are extremely fine. The design is also unique in that the frame and glazing bars are separate pieces, which makes painting easy, and there is a rear insert to hold a piece of glazing in place. I've had many compliments about the windows, which is a good endorsement.
Have a look at www.jacksonsminiatures.com/bottomrailway.html for the garden railway buildings
www.jacksonsminiatures.com/24_003.html for the doors and windows (and www.jacksonsminiatures.com/24_002.html as well), and
www.jacksonsminiatures.com/24_004A.html for the textured sheets.
I hope they give you some inspiration.
Steve