Best post I've seen here - except that in our area nobody will bid reasonably on one day's work because hauling their machinery takes so much work that by the time he starts working the day is over.
So I'd ask a forestry service- not a landscaper - for a quote for a couple of days work. The local foresters here don't advertise, they are found through the State University which has a Forestry dept.
At this point, we don't know much about the OP's land and he knows little about the capabilities of the tractors he is considering - except he may be coming to realize that 25 hp is not enough, and 40+ is more realistic. He may not have even operated one of the ones he is considering on a similar job.
A few days work will give a lot better idea. Then tthe OP can rent a tractor for a week for clean up. That would be a cheap education. The normal beginners mistake is to think that a tractor is a bulldozer...it is not. Tractors lift and pull things. They don't push all that well and are unstable on rough ground.
Everything depends on the land. Clearing land is brutal. Maintaining it is easy. Twenty acres is a lot of land here...It's almost exactly how much we have, and requires a machine with weight and muscle. We have a JD 310SG that is loaded. It is barely enough machine to keep up with the place.
Luck,
rScotty