As a general rule, you never get out of an airplane what you put into
"fixing it up" unless you can do the work yourself. Avionics in
particular only retain 50 cents on the dollar invested. Engine
overhauls do better, as do cosmetics, but you never get 100% back in
terms of resale value.

Much cheaper in the long run to buy the plane you want. If you know
exactly what you want, however, it'll take a long time to find it.
Used aircraft are like old houses, with weird add-ons that make you
think, "What was he thinking when he had that installed?".

The only advantages to fixing are that 1. you do get to choose your
installer/overhauler, and that does make a difference in the quality
of the result; and 2. you get to choose exactly what equipment you
want.