Choosing a macbook between 300-400 bucks! Help!!!

Hi everyone! I'm searching in the classifieds something to do some FCPX for my YouTube channel. I've got a list with some interesting macs and I need to know what would you PRIORIZE.
- Processor (Core2Duo discarded of course).
- Ram (Post '12 Soldered 4gb discarded of course)
- Video card (Under 512mb discarded)
- Hard drive (mechanical discarded).
- Screen (13", 15", retina?).
All retinas (or iris) within the price are broken or have 4gb ram.
I imagine it's better an older i7 16gb with 512mb than a newer i5 with 4gb or even 8. Am I wrong?
Thanks a lot in advance!!!

Others can help you more about the processor speed but I wouldn't waste money on an expensive internal hard drive (SSD or spinning disc) because you should be editing from an external drive anyway. Just buy a USB3, USB-C or Thunderbolt external drive and go. Personally I would be concerned about long term media storage if your editing machine budget is only $300-$400. Where will you keep all of the files you generate? You might be able to cut your first video but you'll have to throw it away to make room for the second. As far as screen size, I would hate to edit on a 13" screen but you could manage if you're eyesight is sharp. Anyway, best of luck.