Very surprised you have been quoted "silly money" for a hawkeye WRX box as they go for silly cheep money due to the different clutch set up, they have a pull clutch. Think there is a lad on scoobynet been trying to sell one for a while and cant shift it because of the clutch scenario.
What is the code of your box?. From memory I think they are TY754VG6AA or 7AA. The problem is that I think the 6AA is the normal pull set up but the 7AA is def the opposite push clutch set up. If you are lucky you will have the push clutch as these boxes go for around £200 - 300 as they are a pain to retrofit to pull clutch cars.
Can you check the code on your current box, also try and see if there is is a round plug with a 6mm allan key slot on the box, this is located on the LHS of the box just forward of the starter. If there is no plug then you have a pull clutch gearbox.
If you are changing the box coz you def want a six speed then i would suggest you get a UK 6 speed which uses a 3.54 rear diff. That way you can retain all your rear running gear (hubs, shafts etc) as 3.54 R160 diffs are easily available, all newage bug and blobeye WRX's use this rear diff.
This would mean you would only need the gearbox, front shafts, clutch slave cylinder, flywheel and clutch, linkage and propshaft and an R160 3.54 rear diff.
If you go down the route of using an R160 rear diff you need to redrill the diff flange as the 6 speed prop uses a different PCD, easy job which takes about 10 mins.