Would I see much of a difference between 7200rpm drives and 5400rpm drives over usb 2.0?
Does this mean you are only looking for 2.5" (laptop) drives and enclosures? Unless you're looking at one of those "Green' drives, AFAIK, there are no 5400rpm 3.5" drives.
If you want something smaller like a laptop drive then something like those WD Passport drives is probably a better bet. It'll be more solid than most DIY enclosures and the newer ones will run off USB power alone I think.
1. Buy a decent internal HD
2. Buy a decent external enclosure
From what I have seen for prices of "external" drives, my suggestion above is the cheaper and better way to go.
Just my 2 cents.