I will preface this with saying that as with anything else, you can spend as much or as little money as you want on your seat. The install is fairly simple, which is just like the original seat cover, but depends on the condition of the seat pan and the foam. Take a look at your seat pan and check if it is rusted or cracked at all. There are small triangular pointy catches punched into the seat pan edge metal that you stretch the cover over and hold the cover in place. Hopefully these are all in good shape and not rusted or broken off. This can be fixed too, but there is always that sweet spot where something is fixable, or if it would be the same amount of money to buy a used one in better shape.
Another part that may give you trouble are the chrome metal buttons for the seat strap which screw into the seat pan. The cover I recommended provides a new seat strap, but not the hardware. I have heard that you can find new hardware on Ebay or elsewhere if you keep your eye out if you want to go that route. The buttons on my seat pan were so rusted in place that I had to drill them out. I bought some chromed pan head hex nuts and bolts from Menards that looked nice and fit into the divet on the buckles.
I didn't catch if this is a running machine. So, the only other thing I recommend for you is taking apart the carbs and cleaning them (and do the 3000 mile maintenance too). The extent you disassemble them is up to you.
http://s581.photobucket.com/albums/ss258/accolay_accolay/1977%20CB550F/Carbs/ It's not hard. Use brand new HONDA gasket sets, and use the jets and needles that came with that carb. Don't bother spending money on new jets and needles unless there is obviously something fishy a PO has done.
Good luck and enjoy that 550F.
