fluid: Dexron III, look for "Approved for the H-Specification" on the label
spin on filter: Use a GM/ Delco one from the dealer
filter in the pan: dealer also
(the pan gasket is reusable)
As for doing it yourself it's not hard as long as you are mechanically inclined. There are a couple different ways to svc the trans on an Allison. It all depends how many miles and how much towing you do.
Drain and fill
Drain & fill w/ a spin on filter
Drain and fill, spin on filter, plus a new in pan filter
or a complete svc: Drain and fill, spin on filter, plus a new in pan filter, AND a flush using the machine (dealer or trans shop only kinda svc)
Important tip: when you take the spin-on filter off, make sure you remove the magnet from the old filter, clean it, and put it on the new filter