1. If I get an external harddrive, I could save stuff for cubase on it, right? I want to be able to still have all my recordings easily accessible when I upgrade computers.
2. On a similar note, if I buy cubase, I can install on more than one computer, right? Some things have limits on that.
3. How big of an external hard drive should I need purely for recording?
1) Yeah that's not a problem at all.

2) To my knowledge, you generally pay for the license for a programme rather than the disc, so unless you buy more licenses you can't install on other pc's.

3) Depends how much work you're doing but tbh, the price difference is that small, you may aswell get 512GB- 1TB, they're useful for keeping music, pictures etc on, especially if your disk space is running low.
Yes you can use cubase on more than 1 pc, the licence is stored on a usb protection dongle so as long as thats plugged in, you can use it on any pc.