Digidesign has made PT more versatile.
The Digi 003 is a good pick. The only difference between the rack and desktop version are the controls. The desktop is a controller and audio interface. You can also use it in Standalone Mode. Standalone Mode enables you to hear your audio without turning on your computer.
An ilok key stores authorization codes to use software and plug-ins. Without the ilok key connected, you will be unable to use the software and/or plug-in. Certain software companies use ilok keys to reduce or prevent hacks.
If you plan on doing a lot of traveling and want to save a few bucks. Check out the Avid Mbox Pro. Its basically the firewire version on the Digi Mbox. Depending on your needs it has fewer inputs, outputs and mic pres.
Check it out:
http://www.sweetwater.com/store/detail/PTMboxPro/