Hello Bryan,
As a DJ/Producer who's recently gone professional, this is exactly the same question that I was wrestling with about a year ago.
The clearest answer I can give you is to think beyond customizing a PC laptop, and that beyond question the Apple MacBook Pro is the industry standard for laptop DJing.
Overall, the Mac OS X operating system is more stable, and squarely aimed at creative professionals. The MacBook Pro also has great build quality (it's solid aluminium) and they look cool out in the clubs.
It is possible to DJ using a PC, but as you rightly note, what is more important is the software and external soundcard.
I don't know of any laptop where you can listen to the 'cue' track without interrupting the master output to the mixer, until you add some form of external soundcard.
I personally went down the 'money no object' route. I got the 15-inch top-of-the-range MacBook Pro, which size-wise is the best compromise between screen real-estate and portability. Indeed get the largest and fastest hard-disk drive you can, otherwise your brother will need to start carrying round an external hard drive.
The external sound card does make a difference to the quality of the audio going to the club's mixer and soundcard; although the previous post is correct that good quality sound files (aim for MP3 files at 320kbps) are also very important.
The sound card that was personally recommended to me by DJ legend Sasha at the Miami Winter Music Conference is the Fireface 400. This is pretty expensive (about $1000+) but is the best possible in terms of sound quality and flexible audio connections. Bear in mind that this uses the laptop's Firewire connection, rather than the usual USB connection.
However, something more basic, such as the Native Instruments Audio 8 DJ (or even the Audio 4 DJ), is less expensive, more portable and is still a 24-bit sound card and more than fit for the purpose.
Unless your brother is envisaging some complex live future setup, you're right that the only thing this soundcard will be used for is connecting the cue headphones and feeding to the mixer.
You then need to think about DJ software. Personally I use Ableton Live 8, which is the choice of the real pro DJs who want absolute control, however it does have quite a steep learning curve and each individual track requires a degree of preparation ("warping") before it can be played out live. Ableton is also a professional production software that's great for recording DJ mixes and creating 'mashups'/bootlegs.
Otherwise, the clear choice is Traktor (Scratch) Pro, which is intuitive, powerful and enables beat syncing.
Finally, your brother might want to consider some sort of 'control surface', so that he can't be accused of "checking his emails" when DJing on the laptop! These provide a more tactile experience, since what he can do on the club mixer will be limited by the fact that you're only feeding one single master input to it.
Personally I use the Vestax VCM 600 (another $1000!), which is a solid beast and great for Ableton Live. There are lots of cheaper options (such as the Vestax VCI-100), which will fit the bill if, rather than running several tracks at once, he's simply cross-fading from laptop "deck A" to laptop "deck B".
Well, apologies for such a long-winded response, but my experience has involved quite a lot of painful trial-and-error, so I certainly hope this helps!
I would also encourage you and your brother to check out my free DJ mixes at:
http
/www.MaceCast.com
Good luck and let me know how you get on - I'd love to hear the results!
Thomas Mason, London
thomas.mason@gmail.com