I also believe that if you are not Apple Certified and perform work on a customer's machine that they run the risk of having their warranty nullified.
As mentioned, most new Macs do use industry-standard components, the trickier part of diagnostics is the actual disassembly. Most of the systems are designed in a very proprietary manner and definitely require skill and training to disassable and troubleshoot. The most common actual "repairs" are individual component failure or misconfigured software.
I think you will find that working on Macs is as challenging, but also a heck of a lot more fun.
The recommendation of iMac is a good one, you will definitely want to look at getting an older machine though, as the most recent do not have as nearly as many (known) issues. Althogh Apple has made PowerMacs very accessible machines, so you may want to start there. It depends whether or not you see yourself as catering to a more Professional or Consumer level market.