If you can get an XDA for that price then go for it
If you can get an XDAII for that price the GO FOR IT
Doesnt matter about the contract... if your getting it for £100
Remember there is nothing to stop you going for that contract and then going down to their cheapest.
Personaly I doubt it is an XDAII ( no offence)
but if it is then jump at the chance to get it for £100... just make sure its all legit
I know O2 were selling the old XDA at a discount (to staff) last year because they couldnt shift them fast enough... but I think £500 is a tad large a discount.
Good luck though,
if its genuine I'm jelous... if its not... I'll laff
Developing The 4th Dimension