I think if you could bring someone who knows about piano to see the piano for you, you may be able to find a good quality used piano. From my experience, I felt that the sounds quality is better from those old Yamaha pianos. The pianos made in recent decades are machine made, while the old ones were more hand made, and so the quality of the woods and parts they used had to be far better.
Having said that, good quality pianos always come at chances only. U5 is rarely found in 2nd hand market as you said most ppl who bought U5 were advanced players so seldom that they would change it unless they headed towards something that was even of higher standard. You would be able to find more U1 and U3 in secondary market.
So are you able to wait? U5 is really rare, you may have to wait for long time to try one (don't go to the famous 2nd hand shop in Mongkok) . Furthermore, prepared yourself with appropriate knowledge on how to identify the authenticity of a used piano. Buying a used piano does run certain risks, say if the parts ruined out after you bought it, the serviced company may not be able to replace it with the genuine Yamaha parts etc.