Biggest piece of advice, is don't rush into anything...unless its a screaming bargain, in which case, tell nobody and hand over the cash and get out of there!!!!

You are in Melbourne i see, so why not go to a dealer and play a few games.  Normally dealers are more expensive than collectors, but if you want the safety of a warranty (they may offer one), then it may be your best bet.  Alternatively, get to know a couple of forum members and play their machines and see what you like/don't like about a machine.  Or go visit a few machines on site and play them.

Pretty much any pinball can be fun.

My advice would be to buy a lesser value machine to start with, learn a little more about them, then either trade up or add more to your collection.  If you had a budget (i think its best to specify that first) or say $5000, i would not go out and buy a single machine worth $5k, getting two $2-$3k machines will most likely be more fun for a new collector.  This also has the benefit that if one has some issues that need sorting at some point, you can play the other.  Or down the track you can sell one machine, and still have one to play while looking for another.

Alternatively, if your budget is around $3k, you are unlikely to get an Indian Jones or Twilight Zone unless they are in poor condition.

Many ebay pins are overpriced, and i would be very hesitant buying your first machine sight unseen - or at least without another forum member/someone with experience looking at it.