I always suggest finding local collectors to start. You'll need that help for when something goes wrong and you haven't quite learned how to troubleshoot it yet. I also always suggest finding an early solid state game (either numeric or alpha-numeric) that is 100% working. Meaning all the switches work, and the mechs work, all the lights and displays work. The reason being is that when things break, it'll be a little at a time, and you'll be able to do more of the work yourself and slowly learn the process of going through your machine. You will find games from the late 70s to early 90s just as enjoyable, if not more enjoyable than the new shiny. Lastly, do research on pricing for specific games you're interested in and you should be able to find a machine without overpaying for it.

This way you'll get your feet wet, learn upkeep skills over time, and not have too much money into the hobby if things don't work out.