What's he need one for Jack, they're pretty robust and not difficult should you need to do any work inside.
There's normally only two things that tend to need attention: The synchro rings or the shaft seals - the first one let's you know it's in trouble when it becomes difficult to select gears (#4 and #2 is favourite) they're the ones that tend to take more abuse. Shaft seals leave a oil slick on your drive
See if you delve a bit deeper mucka, it probably only needs a bit of TLC.
A replacement box is no guarantee that it's gonna be trouble free.
ATB
George