the fastest i5 or i7 you can buy will be the best, don't even mention i3, that thing is 2 cores, you need asm uch cores and as much threads as possible,m those are offered by i5 or i7, i7 specially, 4th generation? used pc? that depends on your budget
for premeiere, you don't use that much the gpu, the rendering process is done mostly by the cpu but a fast gpu makes esier the work process, system will feel faster
if you want to make some effect on adobe after effects, it is better to get a nvidia gpu, thanks to the extensive suport of the cuda technology offered by nvidia on adobe suite
so get, minimum a nvidia gtx 750ti if you are on a budget, from ther, as fast and as much cuda cores as possible, a 1070 is good for this purpose and is not that expensive
something you should have is lots of ram, 8 gbs minimum, when rendering, the machine can use up to 12 gbs, if it doesn't find it, it will slow down the process, so get 16 gbs of ram
also two hard disks will help you, one with os and the other with the project and the files, the resulting file can go into the hard disk os or a external hard disk or the same hard disk where the project is, but the idea is not have only one hard disk, you will see the difference and the responsiveness of the pc while working and processing videos
the minimum you should buy is a i5 6500 or similar form previous generations, 16 gbs of ram, a gtx 750ti and 2 hard disks, ideally one ssd for the os and for the resulting files from the project while you keep ad 1 or 2 tbs hard disk with the project files and folders
if you have a very limited budget, a amd cpu can do the job too, but it will be slower, definitively usable but slower than a intel, lots of youtubers with tight budget uses a fx 8350 for this same process, intel is more expensive
for example
https://www.youtube.com/watch?v=ncVidbvDtEQ
in that video he uses cinebench r15 to test a cpu performace, he gets almost 600, a good intel gives 800 or 1000 but the cost of those is 2 or 3 times the cost of that 8350
it is just a idea, in those cases where the budget is tight and find used decent parts is a problem