The important seals aren't that hard to change.
There is like 7:
clutch push rod, and in put shaft, (inside the bell housing both easy and important)
2 drive flange seals, (easy if you have the special tool, and important),
O-ring on the speedo cable, (FFFT)
shift lever (high on the trany, a nusance leak only),
and clutch lever shaft (never seen one that leaks).
Anybody have a non-official-special tool procedure for changing the drive flange seals?
If you don't mind checking and topping off the trans oil, just do the two seals in the bell housing and roll the dice.
I'd be more concerned with corrosion inside the gear box. If it sat in an un heated garage for 10 years, it could have 10 years of condensation in there. Demand to have it drained, and personnally observe the event. If water comes out, politely pass on the opportunity.