200tdi Defender gearbox - How easy is this?

Hi all,

seals / gaskets appear to be giving way on my gearbox and transfer box
(lots of oily sh*t on the outside of the casing).

From inspection i think the main gaskets between bell-housing, gearbox
and t/box are all ok, problem seems to be at the top end as oil is
running down to outside.

Both driveshaft oil seals also need replacing on t/box but im not to
concerned about those (easy job I think).

I think both gearbox and t/box are original and have done 145,000
miles.

How easy a job is it to get the relevant gasket kist and replace the
top end gaskets? Can it be done from inside the car without removing
the gearbox? i.e. just remove the tunnel and seats?

How long should it take a half competant DIY mechanic (me)?

Any advice appreciated.

Thanks
Jon