If you cut the floorboards on an Intermountain car, there will be little structure left. These are very thin plastic models, and properly built they should stay in a display cabinet.
Decals are simply the easiest thing to do in the model hobbies - we were doing them on the old Strombecker models in the fourth grade, with very good results. Now the challenge is not applying them, but buying them. Nobody makes O Scale decals any more - you special order them from cottage industries. Ten bucks plus postage, unless you want original art.