Welcome to the forums!
looks like a fair bargain.
there are a couple of 'common' fault with the miggies but nothing that serious I dont think. check your heater bypass valve for coolant loss as from what I can tell that is number 1 on the failure list. small black plastic thing with 3x 1" hoses and one tiny vacuum pipe.
on the V6's it resides at the very back of the engine behind the plenum underneath the wipers. pig to get to if it needs replacing but cheap enough.if it's leaking it will be all pinky-white and crusty.
give the exhaust a good looking over as if it has not been replaced already it will be getting towards the end of it's life (8 yrs old u know). expect a £300 bill if that all needs replacing.
check the brake disks for wear - that'll put you back £100-£150 to replace all the brakes - it did me :S
you wouldn't automatically think it but check the tyres. if a car costs less than £800 and all 4 tyres need replacing then you're looking at £80 each for decent ones. that could put the total price up over a grand.
also on the tyres check for wear and the extreme inside and outside edges. 3 inches of wear on the outside means bad toe adjustment (cheap) or wishbone replacement (fairly cheap) 2-3 inches of wear on the inside means camber out of alignement. expect to pay £85 for that to be set up - plus a trip to west london to get tony at wheels-inmotion to sort it for you. It has been known for badly adjusted camber to cause blowouts at high speed (not that you'll be going this fast but I heard of one bloke blowing out 2 front tyres simultaneously at 120mph)
other than that just check what you'd always check... before starting the engine turn the ignition on and switch off EVERYTHING that makes noise, fans, stereo etc. then start her up with the bonnet up and your window down so that you can listen clearly for any knocks and rattles. check all electric windows work, electric mirrors, electric seats etc etc. everything electrical that moves needs to be tested as they are going to be most prone to wear and failuer and will likely cost more to replace.
don't discount the car if you find stuff wrong with it, just factor in the cost of fixing (if the problems bug you enough) and add that on to the price and decide if it's still a bargain.
oh and congrats on finding the car that you will come to measure all others against! Looks very very shiny!