Depends what he wants to do and what he already has.
As notmeatall3 said NAS is the cheaper option in both initial outlay and running costs. However, at least as far as I've looked into them, there are no file "ownership" options with NAS boxes, i.e. 2 or more people could work on the same file at the same time and the last one to save would overwrite the changes the other people had made.
If the printer has a network port you can just plug it into the router, but you'll then only be able to have 2 PCs wired to the router (as the other will be the NAS/server). A wireless bridge is another possibility for the printer, but they require power and is an extra outlay. If its only got USB, then it'll have to be connected to a computer that is always on or require that your NAS box is able to share USB devices.
Personally I'd not use wireless for desktop/tower units, but this would require the use of a switch, cat6 cable and trunking to provide protection for the cables (and to hide them) - more outlay and again powered. You can then either switch the wireless off, or 'save' it for use with laptops/pdas/compatible moible phones. Even then WPA-PSK is the minimum security he should be running and MAC address filtering is highly recommended.
Unless your mate's buisness is in image editing (photographs, film, CAD etc), I don't think your mate will need the quad cores discussed in that thread. See
this post of digicol's for self build, dual core, base units for ~£150 each (would probably be worth phoning ebuyer to see if they'll do a bulk deal).
Don't forget he'll need monitors too. Cheap 20" w/s TFTs from PC World and ebuyer here:
https://www.digitalworldz.co.uk/index.php?threads/174278/
Then there is the Dell option. A Dual Core, 2Gb DDR2, 80Gb HDD system, with cheapo keyboard and mouse running XP Pro is available for £119+vat&del.
In todays litigous society some level of ergonomics with the keyboards and mice are a must. Due to cross-talk issues and battery life I'd avoid wireless keyboards and mice in an office environment, but I believe bluetooth is better and draw less power than the old RF style. 'Kits' can be have for less than £30 each, e.g.
http://www.ebuyer.com/product/108204