Big issue with older ktm’s is parts supply ,stuff like exhaust headers are no longer available,
640 can suffer from,worn/rattly clutch baskets
Sprag clutch can fail,
Fuel pump failure,
Rocker arm wear/failure.
Coolant pump spindle can wear and leak.
Instruments can fail on the adv model ,kerrrching if you can find a decent set.
Also the engine can be a right vibey bugger ,tho some are better than others.