If you bag an audio 10 you'll probably get traktor pro 2 with it. The confusing bit is that you need traktor SCRATCH pro 2 to be able to use timecode CD's and vinyl. This is the expensive bit.
There are various upgrade routes from the standard traktor, all of which will probably end up costing you more that you care to spend.
The cheapest way to get 2 decks going on traktor is with a Traktor Scratch DUO package. This is a cut down version of the software that only supports 2 decks and has less effects, etc. I know plenty of people that have run with this live, no problem.
I prefer the fully fledged version though, simply because I can rock 3 decks with it and the audio 8 is a cracking soundcard with more than enough inputs/outputs.
Things to bear in mind:
If you buy a separate soundcard you'll need to fork out for multicore cables and timecode vinyl separately, thats another £70. Definitely get the multicore cables, theyre legendary.
There are loads of different soundcards from NI out there, the old stuff is the audio 2 (for internal mixing, one stereo pair out), Audio 4 (2 external decks) and audio 8 (4 external decks (with scratch pro)). The new tech is the audio 6 and audio 10. The new soundcards require a power source greater than USB can handle, so need another plug socket. The old stuff doesnt.
What I recommend is getting a second hand audio 8 & traktor scratch pro package from ebay. Get the software lisence transferred over to you then put down the extra £70 to upgrade the software to the latest version of traktor.
Also, get a Kontrol X1. The best £150 ive ever spent on a piece of midi hardware.