Even for $500 I'd say it's a good deal, but $300 isn't bad. I paid $300 for mine, and it was "chopped". I put that in quotes because some PO moron put on 8-over upper tubes and 2" drop blocks, a gigantic tractor-seat K/Q seat, sissy bar, the like. It ran when I got it, although it needed a carb sync, and the front brake was frozen open.
I'm wondering if he thinks it needs a head gasket because it's weeping oil out the cam tower ports. The stock head gaskets for these bikes are very strong, I'd find it hard to believe the 550 would be so misused that it would blow a head gasket before something else happened first. Replace the cam tower o-rings with Viton o-rings, and never change them again.
Those side covers look like the Maier covers commonly available from JC Whitney- not as nice as NOS, but good enough, and the mushroom nuts keep them from blowing off as you approach the ton.
The rest of it looks to be in decent shape. My vote is to go for it, and sort out the niggly bits as you go.