I've remodelled and painted a few of the ERTL cow town buildings for western games. I decided to base them on plasticard that was flush with the walls, so that I could sit them right beside my other buildings, with no gap in between. I also modelled the interiors - craft sticks for the floor, internal beams to cover the corner joins. Definitely glue them together - it makes them much stronger - but leave the roof free to lift off.
They take paint very well, except for the soft plastic doors, which the paint can rub off from if you're too rough with them. Prime the body of the building, though, and the paint takes with no problems, and with drybrushing the detail comes out in a very pleasing way. I'll take some photos of what I've done, and post them up, if you're interested.