Great advice all, thanks!
There are some parts of Oregon that are sponge-like but not here. I'm in a banana belt, surrounded by vineyards (we have 3 wineries in town, and a herd of buffalo). The most pressing need I'll have is for ventillation and coolness in the summer (not THIS summer but a normal summer).
I'll be checking out the new more soundproof building materials that are offered now, for finishing the inside. If I can get it to sound good inside, with acoustical treatment materials like bass traps and such, I'll use it for recording too. I don't want it to be too lively, acoustically, or too dead.
I may also be able to squeeze a bigger size shed on that spot ..... Like 9 x 12.