What PC configuration should I buy for After Effect and Premiere Pro

for your budget you would be looking at an intel i7 4 core build. if you want amd you should wait for amd zen to come out, but that might be a few months away for the 4-6 core zen cpu's. amd zen should be much better performance and value, but we will have to wait for reviews with adobe software. there is also some risk with using amd cpu's with adobe software, as it appears all of the devs and QA people use intel cpu's, leading to amd cpu bugs, and that could easily happen again with amd zen...  also, after effects is being rebuilt to make it fully 64bit/multi-threaded. currently it only uses 2-4 cores and the intel i7 4 cores are the fastest with after effects. once they finish modernizing after effects it should use more cores like premiere.

after effects and premiere should be ok with amd gpu's. there are a few plugins and programs out there that only use nvidia, its kinda rare but still a few, so you would need to check against compatibility with any other software you plan on using. nvidia cards have a reputation of working better/faster with adobe, but a few people have reported solid/fast performance with the new amd rx cards.