Im gonna be the devils advocate here and say (once pool in use) test the heater first or have it checked by a tech. Could of been a bad paint job? Ive seen plenty of working heaters with discoloration...

Maybe its just me, but cant tell if people are saying its definitely dead or if it is dead, then its not worth repair. I know the heat exchanger is often is 2/3 the cost of a heater, so most dont repair one.

But not a pool guy, just owner that has ended up helping install/repair/trobleshoot quite a few pools, spas and heaters. Heater swap isnt that complicated. Depends as much on how much you have to change the piping, if you have an automated controller and how comfortable you are with those.