Clever Models (http://www.clevermodels.net/) produces some nice, highly detailed kits and texture sheets for printing on card stock. I have found them to be easy to modify. Here are a few of their kits that I have modified. These models are in HO scale, and were made about three years ago for a module I built for a club layout. The layout has been taken down and set up again a number of times, and the buildings have been bumped against and leaned on by club members making repairs and modifications, so only some of the imperfections visible in these recent photographs are from my assembly of the models.
Here is the Brick Garage (S12). I built it with an alternative roof (provided with the kit). Since I was placing this model in a narrow space, I moved the attached shed from the side to the back of the building. As this placement put the shed door on the side of the shed away from the back door, I moved that door to the other side, and covered the side where the door was originally with tar paper texture. This was a relatively minor modification, and it suited my purposes.
This building is adapted from McElder's Bicycle Shop (S20). This also had to fit into a narrow space, so I left off the loading platform. The setting of the module is northern Florida around 1960. A wood frame building on a stone foundation would be out of place for the setting, so I left off the foundation and set the building on piers.
The last kitbash I'm presenting here is Habbaker's Garage (S47). I decided that the building would fit in better on my module if it were squared off. I printed an extra sheet of the side and back walls, and fitted pieces together to expand the corner. If you look carefully you can see that I used a second copy of the lower roof and rotated it to make a square roof. As I mentioned above, this module is set in 1960. Habbaker's was built in 1920, so I decided that in the late 1940s or early 1950s the garage was prospering enough to expand. I built an addition using cinderblock and roofing paper patterns from Texture Collection Vol. 1. The doors are from the Habbaker's kit.