Wotcha and welcome Nick - you should get a serious piece of kit for that money potentially a widetrack STI blobeye - but that may mean waiting for the right one to come along, the one thing I will say is be prepared to travel for the right car.
Those cars both look tidy on the face of it
Prodrive packs vary dependant on model but can involve a remap, back box, high flow filter, possibly a high flow fuel pump, uprated antiroll bars. They were dealer options in UK but you can buy the packs yourself to modify an original and then get a remap
a couple of widetracks as examples - the black one would have some wiggle room on cost
http://www.ebay.co.uk/itm/2006-SUBARU-IMPREZA-STi-v9-Spec-DCCD-WIDETRACK-www-powerplay-uk-com-/231437348430?pt=Automobiles_UK&hash=item35e2bdae4e
http://www.ebay.co.uk/itm/2004-SUBARU-IMPREZA-STI-PPP-305bhp-WIDETRACK-AMAZING-CONDITION-/111539135670?pt=Automobiles_UK&hash=item19f84024b6
and if you are going to Cheshire motors you'll get to check this one out as well
http://www.ebay.co.uk/itm/2005-Subaru-Impreza-2-0-WRX-STI-Type-UK-WIDETRACK-DCCD-STAGE-1-333-BHP-/251687698252?pt=Automobiles_UK&hash=item3a99c17f4c