I've never seen rusty bike discs. Is that a thing? If you're in Florida, go talk to GC Performance from youtube? He will have relevant insights. He might have goodies for you too, who knows. Could be an interesting project for him to post about.
For this budget, i'd say second hand, but i can understand you like building things. I'd still buy 2nd hand. Rim brake stuff is cheap these days, residual value on deep carbon rim bikes is low, and Florida is flat, so probably safe to run used rim carbon wheels
I have a 105 group on a giant propel on which i've put 15+k km, 11s, as long as it's maintained, it's immortal, but the residual value on parts like these is super low.
8 speed is ghetto.
Are the Conti 4 seasons the same as GP4000? If not, look for GP4000, even second hand? Get TPU tubes for cheap on AliX. The one thing i wouldnt cheap on is tubes & tyres.
310$ for a new frame, rim, shipped, there's a very decent chance it's a POS. Vs a 2nd hand branded bike should be pretty much as good as new. Going super cheap on chinese stuff is a pretty sure way to get crap. They do, after all, sell a lot of crap.