I think some of that would be driven by what comes in and out of your shop. But that is a skill set that takes time to get good at. Specially, if you are building a business on it.
Seems to me AR, 1911, Remington shotgun and long gun, Glock or M & P, and some revolver would cover a lot. If you get those down and understand them well, it would at least put you in the ballpark for most firearms.
Learn and know the cycle of operation. It will help you troubleshoot any problems you run into.
I don't get to choose. I go for all the firearm types in our inventory. I have to go to a lot of manufacturer sponsored classes for work, but most factory certification courses aren't very good. But if you are starting from scratch you can learn some things. In my experiance it all depends on luck for what instructor you get.
It sucks because there really is no "school" to go to for that. You could go to gunsmithing school but it sounds like that is more than you are looking for and it takes 2 years. If you were in ABQ I'd help you out. Maybe there are some guys out your way that can help.