For what engine?
Please provide enough information that people can help you out with!!
On a 1.9 TDI I have looked into it to swapping to a 6 speed 2.0 TDI box...
You need
driveshafts
(I think you need the larger hubs as well)
Box
Clutch
Associated gaskets etc
Edit: Stater motor as well, yup!!
It's not super hard it's more expensive. I was looking into it on my old car, as the 2.0 TDI boxes are much stronger than the 1.9 boxes that I chewed through...
You can buy a 6 speed 1.9 box (they are from the sport models) and they are a simple swap and replace (no extra hardware needed) as it's the same gearbox just with an extra ratio crammed in, but they're just as weak and a bit pointless to be honest.