Bill,
I really like Russian Birch plywood (very similar to Baltic Birch.) Plywood Company of Fort Worth has the stuff in 3mm thickness increments (~1/8"). They come in 5'x5' sheets and some are available in 4'x8'. They are not open on Saturdays, though, so you need to swing by during the week.
http://plywoodcompany.com/application/home/main.aspx
I've used both the 3mm (ceiling) and 6mm (upper and lower floor skins) in my build, and I will be using the 3mm on the exterior followed by aluminum. On the floor I used 3 coats CPES, 1 coat epoxy, and (by Saturday afternoon) 1 coat rubberized undercoating. I will probably use CPES on the exterior plywood, because I have some left, but I think any sealer, like a water based poly would be sufficient.
Tom