I bought my first EM machine around 2001 for $25.00. Stumbled onto PBresource and marvin3m by accident, watched a couple of the TOP DVDs, and within a few months, turned a piece of trash into a working machine. Was it perfect? Not at all. I made mistakes along the way, screwed up some contacts, did stupid things like adjusting the bottom of a switch. We all have and things still stump me sometimes (right now I have a score reel that doesn't always carry over that I can't get adjusted perfectly, for example).
I'm by no means an expert and still feel like a newbie when it comes to fixing these machines. Gottlieb schematics still look like hieroglyphics to me for the most part, and I would never work on someone else's machine for fear of doing more harm than good.
If you're somewhat mechanically inclined, EMs aren't all that hard to figure out. You can diagnose many problems with your eyes. If you know how to solder (I still suck at soldering, but can solder a wire), know what a DMM is, don't mind doing a ton of cleaning and are okay with having to make minute mechanical adjustments, you can do it. Getting something that's been rotting away in a barn working again after 30+ years is a very rewarding experience.