What part should I buy to render videos?
CPU's used to render graphics was a very very long time ago. GPU's surpassed CPU's for this almost 20 years ago. You absolutely do need to get a good graphics card, yes. Don't look for anything external - the amount of data transferred in and out of the graphics card means running through an interface will be so slow as to be unusable.
You'll want one of the cards from this list:

Nvidia Titan X.
EVGA GeForce GTX 1080 Founders Edition.
Zotac GeForce GTX 980Ti AMP Extreme Edition.
Gigabyte Radeon R9 Fury X.
Sapphire Radeon R9 Nano.
MSI GeForce 1070 Gaming X.
Nvidia GeForce 1050 Ti.
AMD Radeon RX 480.

And more than likely, if you have never done this before, you'll need to get yourself a new motherboard to support the graphics card. And look at fast disk...