Great post! I was just poking my head in here to ask about the same thing. I had first dibs on a 1994 Aprilia Trials bike for $1500 and waited too long to make up my mind, it was gone in a week :( I'd like to own a used trials bike for under 2K myself.
Personally I think that you would be better off spending a bit more money to get something a bit newer, buying as new as you can possibly afford. I started off about 5 years ago with a '94 Beta which was about 7 years old when I got it. It was knackered, and I spent a lot of time and money getting it going right, and overall I think it was a mistake. I think that if I'd spent a bit more and got a newer bike that I would have improved my skills quicker and also been able to spend more time on the bike rather than working on it.
A friend spent US$500 on an older air cooled Beta, and then spent the price of a much better newer bike getting fixing it up only to find that it wasn't that suitable for modern trials. He also spent about a year working on the bike and rebuilding just about everything on it. He then went and bought a '97 Montesa which again he ended up spending quite a bit of time and money working on it - but the difference was that he did actually get out a ride it a bit more. He now has a 2002 Montesa which he hasn't had to touch apart frrom the usual cleaning, air filters etc and he rides a lot more.
If you really want to be spending your time working on the bike then thats fair enough, but if you want to be out riding it then you probably are better off to go and buy something that will be reliable and enjoyable. I don't get a lot of spare free time to work on my bike, or to go riding so its a much better choice to have something that will start first kick, and as long as I keep up on the easy routine maintenance will continue to be reliable.
I started the family out with a '94 gasgas and a '95 beta. Gasgas was ridden hard but they sure seem to hold up well. The Beta was fairly low hours. Both bikes worked well, but as the whole family got into the sport, we've upgraded. I sold those bikes for $1200 and $1300 respectively and purchased 5 bikes this year. Beating the bushes I found: 1996 JT 160, 2001 TXT 200, 2001 TXT321, 2002 Boy 80 and a 2002 TXT Pro 200. I didn't have to pay any more than $2,500 for any of them and we're all competing as a family now. Talk to people in the trials clubs, surf trials links and don't forget ebay. We're all so hooked our MX bikes are for sale.