Hmm pcdj is one of the faves as is traktor 2. Both require you to either buy them or crack them. However, some people have found that traktor realises its cracked and tells you not to use the program again otherwise it will delete all your mp3s! Its probably just a dodgy crack though. I would probably just stick with pcdj though, its easy to use and has everything you need to get started.
Pcdj fx is the newest version which has a sampler, master pitch controls etc but for simplicity i would use pcdj red. Its just like a half decent cd player, you can set loop points/cues etc as well as doing motor downs/brakes. You should be able to get a copy off soulseek for "trial" purposes of course!
With most djing apps you are going to need either an sb live based card or 2 sound cards. One for monitoring and one for output.