i,m riding kx 500 on daily base and i find it very reliable.
the only thing happened was a view busted sparkplugs,
due too a little bit off search for the perfect carb.setting,
which is done and after that no more fooling plugs
so you bought an 97 er ?
there is a few things to check if you want
is the subframe not cracked
is the exhaust properly mounted and the muffler,s inner perforated pipe ?
its often that they come loose due too vibratin, jumps ,hits taken...
and you have too set some bolts a bit tighter for not loosing them one after another
these are the week spots on a kx 500 in general