I cant see why you would have any problems. When I have changed many parts they are pretty much like a swatch watch really. Take parts off and swap them around. So cant see why that would be any different, and with the same code. And I believe the old classic shape STI was 5 speed gearbox. But would double check that make sure it's not a 6 speed
And I meant send them an email or ring them and see if they could tell you if you can use any gearbox. But as I said before I would take the risk myself and see if it fits with that one you have found. I can't see there being a problem, but some things like this you never really know until you get the old one out. And why it was out I would also do the Clutch :( And I know you said the £ can't go to far. And believe me I bet there's not one person on here who has not been there mate. But doing the clutch at the same time will save money, and I'm sure the garage will tell you the same thing. Unless the new gearbox has the clutch in it they can check it I guess before hand. :)