Thanks for the questions, yther. You guessed right.
I do only carry a few models that I've chosen specifically because I believe they are high quality. I carry the Dell Optiplex line (mostly GX260, GX270, GX280) and the Latitude line (D420, D620, D820). I try to keep to just those six models. I'll try out some of the newer models as they become affordable to purchase used.
I set up a computer exactly how I want it (with extra software like gnucash and inkscape), then make an image of it using CloneZilla. My CloneZilla images are on an ssh server and it takes under fifteen minutes to set up a new system (if there are no hardware problems). They install over a local network connection. I have separate images for the several of the different models, but the Linux kernel is great in that it will boot up on most anything, even if it was installed on very different hardware.
I want to create an open documentation project around the processes and business concepts that I use in my shop. Similar operations could be opened in every city with a population over 30,000.
A computer repair shop has to work on Windows computers, of course. But I'm doing just fine with only selling computers with Ubuntu (I do offer Windows, if someone requests it, but I haven't sold a computer with Windows, yet).
Please post to this thread if you are interested in the Computer Sales and Repair Shop Documentation Project (I just made that name up).