I'll have a look at my Mk3 cassette body tomorrow for you, now retrieved from storage.
The wheels true up in much the same way as any normal steel spoked wheel, but are fiddly, since the adjustment is at the hub rather than the rim.
The spoke keys are pretty straightforward, except that you need two different keys, either or both splined or hex depending on your model. Hold the spoke still with one key while turning the nipple with the other key. If you don't do this, the spokes just twist and wind up. I think I have a few spare spoke key sets in the drawers.
Spinergy still sell compatible spokes, since they still make 26" wheels for the wheelchair market (where, confusingly, they are known as 25" wheels
Don't ask.). The spokes are sized by the overall lend-to-end length of the spoke.
The rear hubs are often for sale because the rear rims (made by Sun) wear out (and subsequently split around the circumference), and replacements are hard to find in the more exotic 24 and 30 spoke versions. Luckily the later 32 spoke ones are an easy swap with plenty of Mavic, Sun or Velocity rims.
All the best,