I finished a 550 last June that was in slightly better condition than yours. I spent $2600 all in including the price of the bike. I don't know if you can do the powder coating and Brat build pictured for that. I just did a clean up and rebuild of components, rebuilt brakes with new SS lines & MC. Pamco ignition with new coils, wires & caps, solid state reg/rec, build a new modern fuse panel, tires, exhaust, seat cover. I painted myself to a non OEM color and did an engine top end inspection with new head gasket, lapped valves, honed cylinders with ring & cylinder measurement & inspection.
So I have a clean, rebuilt, non 100% stock but stock looking bike that is reliable and a blast to ride.
You can also do what you want in phases. Clean up & rebuild components to get it running. Add some paint even if it's from a rattle can. Ride it for awhile and plan where to take it next.
Above all, enjoy yourself working on it!