I bought an original low mile (7,000) 550 that had been sitting (on its original tires with most of a tank of gas) for 28 years.
I did no mechanical work except to clean the carbs and replace all the seals.
I Installed new exterior rubber (fuel hoses, tires, tubes, grips), spark plugs, and a battery; bled the front brake and adjusted the rear (keeping the 34 year old pads/shoes), lubed the (original) chain, and then put over 4k miles on it over 4 months before (foolishly) selling it.
The miles included a 6 day, 1,400 mile trip around Lake Michigan with a bunch of other SOHC4 members in the first Relay Rally.
I only had one mechanical issue and that was my fault for not retightening something while setting the points.
I ethusiastically recommend the 550 and personally bought a basket case 550 a few months ago I am very slowly putting back together.