grab a compression tester from your local auto parts store (cheapest on is fine) and check the compression before you buy (110 - 135 is where you want to be)
Look at the fuel lines for cracks, old age, tears that have been fixed with electric tape, of course a good track, if studded, inspect to ensure ALL studs are still on, if any missing, you will also throw studs and need a new trac!
When you go look at a sled, and the guy says it starts first pull, he is usually wrong, unless its a newer sled. Before you start it, pop the cowling, and feel the engine and exhaust, if hot or warm, then you know he had the sled running prior to your comming. And thats why it starts first pull.
You will never know how it starts from cold, until you try for first time. maybe 3-4 puls or 15-20, but as long as it runs, there isnt much to go wrong with an older 2 stroke. manule primer too is nice!!!
I just got a second sled a 92 mach 1 617cc, it has 4k on it, good trac, good seat, good lines, good clutch, mint sled and i paid 1300 CAD$$.
Remember a newer used sled could have more things on it to go wrong, like electric start, reverse, fuel injection, etc. all things to go wrong, unless sled is very new, i would stay away from that. (depends on how much you spend)