USB should work fine assuming the system itself can boot to usb (googling it looks like it can, you may have t hit delete to enter the bios to tell it to boot to usb first though)... distro inherently isn't a huge varient in terms of resources used however desktop environment is. You absolutely want something with a very light desktop manager, LXDE, XFCE etc... So depends, if you are looking for something very simple... I'd say probably Linux Mint XFCE edition, if your goal is to learn how linux works, don't mind reading up some documentation, arch linux is amazing.

If you have experience with any distribution or like something, almost every distro has a version that's distributed with a light weight desktop manager. The desktop manager is where the bulk of the system requirement weight is.