You're not new to motorcycles so you already know many of the potential mechanical issues to watch for; 2-strokes should start easy and have nice discernible compression, sound needs to be a nice crisp 'Pop,Pop,Pop' on idle. Issues with bottom end seals, rings or a poorly seating reed valve cages will manifest as harder starting, decreased performance and a less then crisp exhaust note. Rear suspension should not have excess free-play in the suspension linkages when you lift the rear end of the bike, front fork stanchions free of rock damage scars or signs of ignored oil seals and slide bushings, exhaust should be mounted securely and free of excess oily goop leaking out of places it shouldn't. Brakes, chain, levers, bars and foot pegs are largely interchangeable between makes and models so those are usually easy to service, maintain or replace items, internal engine parts not so much and that would be my greatest concern in purchasing a 'rare' model. Even if budget is limited I would be looking for ~1998 or newer units, more then 20 years is a long time for rubber parts to remain pliable and free of cracks even if the bike has been mostly parked. (
I think a lot of the older trials bikes that appear on kijiji in mint condition have spent much of their life parked) If it's not rolling on 1 or 2 ply radial tires then the tires are probably garbage, lots of tread with nice sharp square knobs on a really old trials tire only happens when the rubber has become rock hard and turned too slippery to ride well, expect to pay 3 or 4 hundred on a new set of tires. There are a few models out there that don't fair so well in deep water, usually the ones that have a relatively small air filter located where the rear wheel throws water almost directly into the air intake (
Xispa & early Scorpa models come to mind as notable examples of this) Some models have relatively fragile plastics (
fenders break easy) A few have near indestructible plastics (
1999 Cota 315R fenders were amazing, you could tie them in a knot and they wouldn't snap) Lift the wheels off the ground and give them a spin, rim dents or spoke issues will become immediately apparent. Watch for rear swingarm twist or cracks, some makes and models are more prone to that then others, expert and pro riders tend to test those components to their limit on a regular basis.
Hope this helps some