So many options. I'm sure you'll get a flood of responses here, but I'll make only a few general recommendations and personal opinions:
It doesn't matter whether you go mac or pc (that argument is now defunct), unless you have your heart set on one particular piece of looping software (e.g., Mobius is pc only and others are restricted to one platform or another)
For software, check this out for options: http://www.krispenhartung.com/software-looping.htm (some are free and perfectly fine, others cost little to a lot)
I would go for an audio interface that is firewire (make sure your laptop as firewire too) and also MIDI on the same breakout box (check out presonus, motu, m-audio, RME, etc...or just search on audio recording interfaces until you find something that has those features).
Whether you go mac or pc, make sure you get one with the Intel duo core in it...I'd go for 2GHz and 1gig of RAM at least.