if it's generally in decent shape and running well, for 450 it's a great price. the bike has 2400 miles after 40 years? is that true or some random number on the counter? if so, it's a steal if it's not been rotting away.
turn signals are easy to get. getting an original muffler won't be easy and will be pricey but there's plenty of nice aftermarket stuff if you don't insist on 100% stock. as others have said, some leakage at the top end is common. it's not disastrous and can be fixed.
fmctm, you might be thinking of the rocker shafts? the head covers of the earlier models didn't lock the shafts so they could move along with the rockers, which could wear out their journals. can be remedied by swapping a to a later 550 cover (k3, f2 or late f1). i'm dealing with the same thing at the moment.