I was in the same position as you a few years ago, and I ended up with two utterly f-ed games that took a ton of money and time to repair.
Since then, I've been to a bunch of other auctions both for myself and for an arcade I worked for. Here are my tips:
Spend tomorrow looking around, but don't plan on buying anything. Auctions that are held in December always have games sell for nearly double what they are worth because of the holiday season. I'm assuming you are going to the SuperAuctions auction in Dallas, and I'm sure that it will be no different than the auction that I'll probably be stopping at on the 17th in Milwaukee. I've seen games in Milwaukee sell for $200 in December, and the same type of games sell in February for $50.
Games are best to buy when there is more than one of the same title at an auction. The first game will usually go cheaper, as everyone else just plans on bidding on the next machine.
You said that you don't know too much about the inner workings of the machines. Here's a huge tip that would have greatly helped me out with my first games -- if you see something that looks easy to fix, it isn't. One of my games had a monitor that was slightly screwed up, and there was no way to fix it unless I wanted to pay $200+ for a new monitor. I bought a pinball that looked like it just needed some new plastic parts and a couple drop targets, as well as some care for the cabinet. While I've fixed up the rest of the game, I still haven't found plastics for it and the cabinet's water damage is horrible and not something I'll be able to easily fix. I fell in love with the pinball, so it's all good
Lastly, set a limit for yourself on how much you are willing to pay for a game. Going to auctions is fun, but I've seen many other people overpay for a machine that they end up not wanting. They end up putting it back in the auction at the end and get about 1/2 of what they paid for it. Personally, I write down the games I am interested in a notebook and the amount I am willing to pay for them. I'll let others start the bidding until the amount is close to what I'm willing to pay or until the bidding winds down, and then I'll bid up to my limit. If the game's price exceeds that, I stop myself and wait for the next machine -- or auction to try to get a good find at.
I'd suggest subscribing to GameRoom Magazine and the PinGame Journal (PinGame only if you are interested in pinball) as both magazines are great resources for the hobby. Once you've learned more about the machines, you can get pretty lucky sometimes. I snagged an-almost working Bad Dudes JAMMA cabinet at a SuperAuctions not too long ago for $5.00 and was able to fix it and make it work for another $15.00. With a little bit of sleuth work, there are more deals just like that to be found!