Access to the rear seal is possible with the gearbox fitted in the car.
You'd have to remove the propshaft and find a way of holding the yoke in a fixed position to remove the nut on the output shaft. This isn't a job I'd want to try and do under a car on axles stands - a bit nicer to do on a four post lift I imagine.
I suspect that you will not have the more modern 12 point nut
(
http://www.peachparts.com/shopforum/diesel-discussion/341253-717-412-5-speed-manual-gearbox-refresh.html)
Probably going to be a slotted nut like this one (on an automatic transmission)
(
http://www.peachparts.com/shopforum/diesel-discussion/304903-722-118-automatic-transmission-rebuild-monster-diy.html)
In either case the nut is peened to the output shaft - even off the car working on a bench these nuts can be testing.
I've never bought the slotted socket for the slotted nuts - it is unfortunately a different size for manual gearboxes, automatic transmissions and rear wheel bearings / rear wheel hubs. The replacement nut will be 12 point anyway - you can't get new slotted nuts from the dealer any more - so buying the slotted socket tool is pointless in my opinion. I use hammer punch and a little grinder to remove the peened metal and the nut.
As for spares - the dealer is probably best for just the seal.
If you post up your gearbox type 716.XXX I might be able to show you the way to an Elring or Meyle (yuk) or Febi (yuk) full reseal kit that might be cheaper than the dealer seal!