If you're going to hire it done, get someone with a forestry mulcher to come in. It's not cheap, but it's one step and done. Insist on 1-2in below ground and you'll have nothing left but some chips and raw land.
Doing it yourself, you've gotten good advice already. If you have big stuff that you need to take down, use a big tractor and something like a Baumalight Brushcutt (open back mower). I have one, it's a good implement that's kind of like a poor mans mulcher. Slower but cheaper and doesn't do as wonderful a job. But if you hit it a few times, it'll look "mulched". All depends on how fast you want it done and how much you want to do yourself vs hire out.