I heard from a specialist that the Mk3 gearboxes are notably unreliable, and some said they wouldn't fit a s/h box because you have no idea what it'll be like, they fitted one to a Golf which then came back within a couple of months because that one had failed.
A full g/box from VW will be over a grand, a recon box will be about £600, then you may as well add in a new clutch while the whole lot is out. So add another £100 or so for that.
If you can put up with the car being out of action, you could get your gearbox rebuilt, that way you know it's history better and you should get a guarantee from the company that did it.