Here's the link for anyone else who wants to look.
Protools (avid/digidesign) are notoriously fussy about compatibility of equipment.
They test and verify certain computer systems and publish this on their website.
Now, I never took that to mean that unlisted systems wont work; I took it to mean that you're on your own if it doesn't.
The best thing you can do when buying a PT machine is copy someone else.
See if you can find someone else using that machine with PT.
They do recommend storing sessions on a second drive.
That's easy with a desktop PC. Either add in a second drive for sessions etc, or pick up an SSD for your system drive and use that 2TB one for storage.
80-160gb is often plenty of space for a system drive. It just depends what you do/use.
That aside, why ProTools?
It's probably one of the most expensive and least convenient ways to get into recording.
It's still tied to an ilok, their customer support blows, and it doesn't offer anything that most other DAWs aren't already doing.
If you need compatibility with an other PT user, or you already know it very well then fair enough, but if not I'd reconsider.
For the record, I'm a PT user.