Avoid 1987 models if you can help it. Their CDI is unique to that year, so small chance of finding one cheap when it fails.
2001 or newer models will have a disc brake on the front, while they no longer have a kickstart. I'd say get a 2001 or newer if you can, and you can add a kickstart later if you need it.
Look over the frame for cracks, check the rear shock near the bottom for the weld to be cracked (not likely, but possible).
If you can do a compression test or leakdown test before buying, that would save you from the biggest problems you might encounter. As well, you'll read about the right and wrong oil filters on this forum. If it has the wrong one, it's dead or dying quick. If the oil is low, thin, or the filter is full of metal, it's either very new, getting very old, or hasn't been cared for.
2005~2008 models might have a bad base gasket, leading to a leak. If it has the leak, you can bargain with the seller over that. It's not a terribly hard thing to fix, but if they let the oil run low, that could be a larger problem. If the oil has been leaking but is full of fresh, clean stuff when you check it, there's a possibility it ran low on oil for awhile.
Wheel work and tires are kinda cheap on this bike compared to others, so a broken spoke, soft brakes, or a wiggly wheel isn't a big deal to fix, but make sure it gets fixed.
Check the exhaust for heavy rust or holes. It'll cause the bike to run worse if it gets bad. Bargaining chip.
Pre-2001 bikes have a manual cam chain tensioner, which may have been unadjusted, and 2001-present have an auto one. Either way, pop the left cover off the side of the cylinder/head and see how much play the chain has. You'll see rub marks in the motor if it's bad.
Obviously, a seller might not let you do a lot of these things, so try to take someone who's familiar with bikes, check oil and tire pressure, and get them to ride it a bit.
The biggest pieces of advice I can give you are the first one and the last one there. This isn't a hard bike to maintain, just try to make sure it has a good motor. Old bikes with low miles will need fuel system work, chain, and rubber stuff usually. High miles bikes might need those things, as well as engine work, typically. If you find any of the things I have listed, and the seller hasn't mentioned them before, don't pay what they are asking. For anything engine or frame related, don't pay for it at all unless you can afford a new one.
If you see any ads online for some in your area, post them up here and we can always tell you if something is clearly wrong with it. I doubt anyone on this forum would come to Sri Lanka to steal a good deal from you. Welcome to the forum, by the way.