I just recently bought my first few pins too. Some things I noticed were 1) The prices people said to pay on a here and a couple other sites were not real world pricing today. If you want a game today your going to pay a little more, it's a sellers market right now, or, wait a while until something pops up. 2) Don't get discouraged if some sales fall through, there are some sellers that evidently don't wish to actually sell, just test the waters. 3) Check Craigslist early, people watch that like a hawk. 4) Spend time with the machine, play it, open it up etc. 5) Don't be afraid to say no, it's especially hard if you drive a ways to check it out and/or it's your favorite game. Condition vs. value is highly subjective so if you have a gut feeling it's a bad deal, don't do it. I turned down two whirlwinds, it was tough to do. 6) Be prepared for some maintenance, you will have to fix stuff. 7) Make 100% sure it will fit where you want it, my space was challenging, had to measure a few times to make sure it was even possible to get them there, and it was, but not super easily.
Hope that helps a little, just be patient, but, if you have a good feeling about a game and really want it, it's usually worth another couple hundred bucks for the enjoyment factor. As for repairs, there are expensive parts, but in my limited experience it's been mostly adjustments, and maybe a broken switch, or missing a diode. Pretty easy stuff.