Actually for $500 that aint that bad as long as it's a runner. XJ550s are pretty bulletproof and 18,000 miles is pretty low for a 25 year old bike. Only thing is, cant really tell from the pics but the exhaust cans look kinda rusty, strangely though.....the downpipes don't. Wondering if the exhaust (right side esp) was replaced from a scrap yard after a crash. Without a close up pic though it's hard to tell if it's rust or just dirt / heat colouring.
25 years old, chances are it's been crashed at some point in it's life......the frames on XJs were never the strongest on the planet and have a habit of bending in odd places, look for scrapes on bar ends, mirrors, footpegs etc. If it has been crashed the only way to tell if it's bent is take it appart and put the frame on a jig. Of course, if you need to steer left in order to go in a straight line then that's a pretty good indicator as well.