Pinchflat - I ran a '99 Techno 250 for 4 years which I bought second-hand, but in "as new" condition. Apart from an early problem with the electric "generator" (not sure what the proper name for it was) which needed replacing (common fault and expensive), I had absolutely no bother with it at all and it was ultra-reliable (in fact I've had more bother with my 04 Gas Gas in 1 year than I had with the Beta in nearly 4)
Although a little heavier than the latest bikes a 99 Techno is still perfectly capable of competing at most levels.
The Betas have the advantage of a removable flywheel weight which you could leave on whilst getting to "grips" (pun intended) with trials riding and take off later if you want some more "zip".
All Betas suffer from an itermittent carb leak problem (this can be fixed via a mod -see the Beta forum) - just don't stop the motor whilst bike is pointing downhill or you'll get a sweat on trying to re-start it!
I agree with the advice about local dealers.
I think when buying a bike of this age you need to assess how well it's been maintained. All trials bikes will have suffered bumps and knocks but there are a few things you should look out for to assess how well its been maintained. Check out for corrosion (suggesting not cleaned properly / WD40'd), condition of nuts etc (if rounded suggests slack maintenance), check for play in bearings - wheels, headstock and suspension link, leaking forkseals, buckled/dented wheels and wheel spokes for tightness.
You shouldn't need to worry about the motor as its pretty much bullet-proof.