I have a slightly different take on this. Aside from some obsolete chips (sys 1 and 80). There is 1 huge barrier to entry for beginners: the power supply. I haven't had a system 1 that didn't come back to life with a power supply rebuild and some connector recrimp.

The power supply with the integrated heat sink can be too much for someone without a lot of soldering experience to handle. The instructions on updating these boards to make them a little easier to deal with are straight forward but most pinball repairs don't involve a drill.

In this situation it is good to have 2 things:
1. An aftermarket power supply. Your confidence will shoot way up when you know the machine works and you just need to rebuild the old.power supply.
2. A power supply bench tester. This is a much harder lift. Clay has instructions in his TOP 5 set and it requires the small transformer out of a system 80. This allows you to bench test the power supply and with some creative jumpering you can avoid having to solder in the big transistor to test the out out.

If course#2 only applies if you think you'll work on a bunch of these. Someone had given me a sys 80 bottom board so I spent a morning building a bench a bench tester.