Quoted from TomyAtomic:
How much technical wizardry is needed for basic troubleshooting and upkeep of late 90s and early 2000s games? And are there any specific “popular” pins for new collectors to avoid due to reliability and repair complexity issues?
None, if you have a fat wallet and live in an area with good techs who do house calls.
Or, at least a willingness and open mind. There's a lot of help to be had here, especially for people willing to help themselves. Most games have mechanical schematics that help you see what goes where. Mechanical issues tend to be fairly simple - once you stop looking at the giant box of moving parts and focus on just the mech that's giving you trouble. Soldering lugs, no big deal. Electronics issues, might be a bit more to learn on; but there are people you can send boards to who are experts. Even then, a lot of issues like identifying a blown driver transistor, there are guides here on how to use specific tools, and spotting the bad part is as simple as "which one isn't like the others".
Pinside is a great resource, but only if you articulate what you're seeing, what you've tried, your observations when troubleshooting. Pictures (clear, in focus, well lit) help immensely. The more effort you put into it, the more help you'll get.
pinwiki.com is a fantastic resource, *especially* for WPC-era games. A bit less so for Stern (at least for electronics).
vid1900 has guides here on pinside on just about everything. He's a cranky bastard but his advice and tutorials are pretty solid! I know I own him beer for the trouble he's saved me, whether he knows it or not.
You may find locals who are willing to come *help*. Key word: Help. Learn from them. Don't just treat them like an unpaid tech.
Or, see the earlier fat wallet comment.