In reply to Frankie boy:
Well we've got a 2006 plate W639 111, rust wise to 2006 isn't too bad though I've had to have the rear arches repaired. The panels on mine are partially galvanized, the 2004/5 were the really rusty ones. After 2006 they went full galvanized I think.
The main thing to look out for is the infamous "black death". This is basically injector leak, I had to have mine resealed, it's a job best done at the dealer or a specialist. To check for this you need to remove the air intake box (2 mins) then the injector cover (10 mins). The going rate for re-sealing of injectors was £120 - £150 per injector a few years ago.
The next common complaint is gearr stick socket wear, rhis can be sorted with a 3d printed 2 pice socket bought off of ebay (£10), you'll know if its worn as the reversing lights won't always come one due to the micro switch not activating.
The final issue can be a stiff second gear, this can be aleviated with a different spec oil being used in the gear box (plenty of info online in the merc forums).
My electronic ignition also went which cost £700, though I don't think this is too common. I've also replaced the turbo, and prop shaft, though the van is 15 years old with about 130k on the clock.
Finally, if I rember the automatic gear boxes are best avoided.
If buying I'd look for one of the 2011-14 facelift version W639 111 models (or higher spec). Engine-wise the more powerful Vito's as a rule get better mpg owing to the rear diff ratios. We've got the W639 111 variant the lower speced 109 wasn't fuel efficient enough for me to consider.
Edit:
There are 3 wheels bases, the advantage of the lwb or xlwb that the spare tire is carried underneath the van rather than inside.
Post edited at 14:44