XJ600's where reasonably robust ( or as robust as a 600 can be ) they didn't exactly attract thrashers & hooligans, just make sure there are no nasty knocks or bangs, any top end rattles, no oily deposits in the pipes.
if there's a bit of noise from the right of the engine pull the clutch in, see if that cures it, clutch bearings can get a bit rumbly on XJ's, tis no biggy, even XJR1300's do it, tis a family trait if you like.
Make sure it starts ok, changes gear ok, just use a bit of common sense, the bikes cheap enough to not loose out too much if you drop a clanger.