I think the min. would be a 33000 chassis for a 14 ton crane. If you go up to a 17 or 23 ton, chassis should be 48 to 52k, tandem axle. I am going by what I have read the min. chassis should be on some crane manufacturers website.
Here in Hawaii, boom cranes 14 to 23 ton w/operator get $110-125 an hour with a 4 hour min. A friend of mine bought a 23 ton crane about a year ago and now he is keeping it busy quite a bit. You might very well find yourself getting out of the tree business and becoming a full time crane operator. If it was guarenteed that I would make $125 an hour working by myself doing crane work everyday, I would be out of the tree business before you could blink an eye.