Larry,
The lift is going to be your biggest concern. I'd dig a good sized footer for it for sure. I've never installed one, so you'd have to rely on the manufacturers requirements for that. Whatever they require, I'd add some for overkill.
12 inch centers will work well, but for all that weight, and more important, movement, I'd upgrade that area to 5/8 rebar. Won't cost too much more, but it will give you quite a bit more strength.
Speaking of strength, are you mixing the mud yourself, or bringing in a truck? Either way, you might want to add another bag of cement to the mix.
For a few hundred dollars more, you could pretty much eliminate the cracking problem from the decomposing roots.
Another thing to remember is the roots will decompose allot slower if you eliminate as much water penetration as possible. Roots in totally dry soil will last quite a long time just as they are.