Another way to ask this question: How would I start all over in this pinball market around the same time I started my first real job ( yes , I know you can start earlier ).
First I would start with non working but nice cosmetic condition EM's and Early SS's. In the EM area I would focus on 1960/1970's vintage Gottliebs. In the early SS's , most likely classic Sterns. As stated here, need to learn how to read schematics, learn how relay/contact logic works, learn basics about transistors, diodes,capacitors and simple transformers. Learn how to troubleshoot problems, LEARN HOW TO SOLDER (especially on boards) and spend time on the various websites on how games work /solving problems.
I would buy a non working , good cosmetic condition game, put it in a place I could work on it with good lighting and start the process of getting the game into working condition. Once I got the game up and running, invite over friends, get beverages of choice and set up matches. The older games are great for these type activities.
After a while either sell my now fully working game in nice condition for more money then I payed or buy a second game. Rinse,lather,repeat. I would build up my fleet that way. I would not worry about the three letters, NIB, until the bank account could support.
I am more fond of the games that I have found, rebuilt, than the ones I bought NIB. There is a lot of fun to be had in this hobby without spending money on the new shiny toy ( which of course I am now guilty of .