Not only do you need the "Ultimate" version of Pro Tools, you also need an interface(as mentioned) AND a Digilink license on your iLok. The Digilink license is a one-time purchase($299). As for the interface, you have lots of options here:
Digidesign 96IO, 192 IO
Avid HD IO, OMNI, MTRX
several 3rd party boxes that also use digilink(Focusrite, Lynx, etc)
In case this stuff is all new, here's some stuff to be aware of on the old Digidesign stuff; the 96IO is on par with 003 for conversion. 192 is better but be aware of the different versions; 192 "standard"(8 analog IO and 8 digital IO) and 192 "Digital"(all digital IO so requires converters, so I would skip this one). The 96, 192 and HD IO are all LINE level IO(so you still need preamps to record microphones). The 96 and 192 also require the adapter cable to convert the older DigiLink to the newer Mini-Digilink. The Avid OMNI has preamps and monitor control function, so its a great choice by itself. I can't speak for 3rd party stuff.
I ran an HDN rig for several years(with 2x HD IO and 2x 192 IO) and loved it
(and would do it again in a heartbeat if I needed more than a handful of IO)
Registration to the previous owner should not affect function.