I highly recommend using
Reaper as your audio software. It's much more flexible than Audacity and can do multi-track recording should you ever need to do that. It also has great support for measure-based timebase + tempo changes, which is extremely helpful when using programmed drums. Reaper is more complex than Audacity, but actually very easy to use.
When I record demos with my band I write up the drums in MIDI, which I load into Reaper and run through Addictive Drums - you can specify the mapping of MIDI notes to drums in AD pretty easily and control all kinds of shit - you can also record the drums on an electronic kit and use that MIDI data to power AD. Addictive Drums is expensive, but there's a lot of options for computer-based drum machines out there that are cheaper (plus, you can always, uh, 'try it out'). There are a lot of freeware drum machines that require you to specify samples - if you do some digging you can probably build a good library and use one of those to play the samples.
As for audio interfaces, I really, really like this one:
http://www.musiciansfriend.com/multitra ... controller but it's probably overkill. If you just need one input almost anything should work. I used a Lexicon Omega (
http://www.musiciansfriend.com/pro-audi ... ing-studio) for a while, and that was decent.
Hope this helps!