It sounds like you have a reasonable background to jump into pinball machines as a hobby. Early Bally solid state are a great starting point for a beginner, there are a lot of machines with huge production runs so parts are generally easy to come by - at least for mechanical and electrical "common" parts. There are also many folks with in depth knowledge of this series of game to help you through problems. Another good choice is the Williams System 11 series of games.

For your first game, it would probably help to buy one that at least does something, maybe at least it boots up. Buying a totally dead game for your first might be more than you want to chew for your first experience.

It might help if you can take someone along with a little experience for your first purchase - at least to identify potential problems such as poor hacks. The more someone inexperienced has been in a game attempting to repair it, the more likely this will add difficulties in troubleshooting and correctly repairing a game.

Lots to learn, read up, and take your time. Asking this question was a great start.
Welcome to the madness!

Sounds a lot like what I do to machines. I call it "restore to operation". Basically, making everything work and work well, getting the machine to be complete if it is missing any parts, and making cosmetic improvements. Often this is over a period of time, and has a lot to do with what I feel like working on, maybe just what is bugging me about a machine's cosmetics that I would like to address at that moment in time.

There's a few popular distributors of parts out there. I use Marco Specialties a lot, I have also used Pinball Resource, Big Daddy, GPE, Cointaker and a few others. Oddball stuff that you can't find anywhere else, you can ask about here or search for on EBay and perhaps get lucky.

If you have "money to burn" - so to speak, that can certainly help. Unfortunately I have had to pass on some more expensive parts, as some of the more rare stuff can command some big prices.