Much depends on exactly what is wrong.
The Clutch can be replaced without removing the gearbox from the car. You unbolt it and move it 'out of the way' as best you can. Its tight, but thats how they do it, and it reduces the hours on the job considerably.Info supplied by my Alfa specialists.
Sounds like you have serious oil contamination which is either a failed seal or gearbox casing damage caused by broken components..either of which means gearbox out..which involves removing the wheelarch liners and the suspension leg and gearbox mounting, and its still very tight.
If I were faced with this decision I'd source a secondhand gearbox with good provenance and a guarantee of some kind (money back if faulty sort of thing) and do a swap, but you will need a clutch kit too.
Guess what...I've just had this job done on my 24Valve six weeks ago. I had a spare gearbox (as one does) and I was charged £360 to take the old box out and put the new in. My clutch was already quite new, and thankfully was not contaminated. In my case the pin between the planet wheels in the diff snapped and ground a hole in the gearbox casing...nasty!
I'm not complaining about the cost which I thought was very reasonable considering the hard work and grief involved!!
Let us know what you decide to do (or you could buy my TS off me..new MOT, new front Discs, 72k on the clock, 92 pre-cat model)