How much data do you have to backup? The current sweet spot for external hard drives are 8, 10 and 12TB WD externals at ~$15-20/TB in the U.S. The caveat is that (as I've said dozens of times here and at videohelp.com) is that the USB interface is cheap and prone to failure. Fortunately, you can easily decase the drive and put it into a third party enclosure or use it internally.
PCI USB 3.0 expansion cards are rare, here's one:
https://www.amazon.com/dp/B00ARJN5G0...ing=UTF8&psc=1. The catch is that it's much more expensive than a PCIe card and you're not getting anywhere near the speed USB 3.0 is capable of. Personally I'd just stick with USB 2.0 and allow the backup to run overnight.
Alternately, you may want to consider using a removable drive bay, assuming you have a spare 5.25" bay available. This will allow you get the full SATA transfer speed. You can decase a cheap WD external, possibly voiding the warranty, or buying an internal drive.
Factor in the cost of at least two drives as a single backup drive is good, but a second backup is much, much better and part of the 3-2-1/1-2-3 backup strategy. Three copies of your data, two as backup, one kept offsite.
Edit: Having going through dozens of external drives over the years and having many of the cheap interfaces fail, I'm currently using a combination of removable drive bays and multi drive external USB 3.0 enclosures. I mostly buy WD externals and immediately remove them from their cases to use as internal drives as even if I void the warranty, it's still cheaper than an external. I live by the motto that it's not if any hard drive will fail, but when.
