MiG 21 LanceR
MiG29SMT
Su27SM
...
The MiG21 must be the most upgradable aircraft since the Canberra. The MiG29 upgrade market is very active, and the Su27 is extremely upgradable. Angolan MiG23MLD were upgraded to become quite efective interceptors. It will be interesting to see how the venuzuelan Flanker fleet evolves, at a time when US pressure is making their F16 fleet inoperative.
Note than buying the F35 means that you won't be able to adjust cockpit airconditioning temperatures without a software tech comming from the States with the relevante line code

. OK, its not that bad, but not by much... Remember the iranian F14s. They were expcted to get an engine upgrade as part of a MLU at some point. Good luck with that.
Cooperating with the US industry is easy only when they want it to be.
Try using the F35 in the event of your frienship with the US turning sour...