Reply to 13 yr old just getting started/What soundcard is best for Numark Mixtrack LE

My son is just getting started, he's 13 and has been playing with several DJ'ing programs like Virtual DJ while we sorted out if this was something he was going to stick with. Now that its been 2 1/2 months and he seems committed I'm trying to find out what he would need to get started. I just placed the order or his Midi controler, Numark Mixtrack with Virtual DJ LE, and of course,now he figures out, after winning the auction that this doesn't come with an internal soundcard and that we have to buy one. What would you suggest for someone who is just getting started? Something inexpensive but good quality that will do what he needs and last I'm ok with used as long as it works. I'm a single parent with a very limited income and I've already spent more than I planned on the MIDI so now it is starting to get more complex than he first explained and is costing more than he said it would. As far as computer, I'm sure it will have some impact and he'll run to me saying he needs a RAM upgrade or it wont work, he will most likely be using my Dell Studio XPS 1640 laptop with (3gigs of RAM and a core 2 duo) or his Toshiba Satellite (2Gig Ram, Intel Celeron processor). He will be doing school dances and friends parties. Any suggestions are welcome but the Midi is on its way via Ebay so we're kinda stuck with it. Also what other things will he need? Anything/companies to stay away from?