Cadre, you asked some good questions, first of all, i will say landclearing was the most enjoyabe thing ive ever done in my 20+ year career as an operator, i cleared right of ways for pipelines for 4 years, quit cause i have young kids and the job kept me away from home, but to try to help you answer some of your questions,
size of hoe is critical, but cannot be answered with one specific machine class, it all depends on what type of clearing you want to do, how big are the stumps, hard woods or pine, grubbing is clearing brush and small trees, so the scope of the job depends on the machine required, if you have a small machine with large stumps it will kill your productivity, yet a large machine in a small area may do large amounts of destruction that may require you to pay to fix, when i started clearing we had all 320 size machines, when i left we upgraded to pc290's, tried pc360's but they were a pita to move and we required moves quite often, the productivity was amazing over a 20ton class machine, plus we fitted 2 of the 290's with rotobek grapples, wow what a difference
consider if timber is standing or hand cut, how to top trees and sort marketable timber, how to stump and what to do with the stumps, burn, bury, grind, haul away, stumps dont burn very well, most people dont want stumps buried, grinders are very expensive, and so is trucking if u find a place to take the stumps
Tops, you can chip them, if u have a chipper
Look into a forestry mower, they make attachments for hoes, although they require very high flow, also have attachment for skidsteer, and companys like cmi out of canada make 600+hp track machines with available stumping heads($650,000).
if your gonna start, start small and i would suggest a hoe with a hyd thumb is a must, and a forestry mower, hoe size agian depends on how big job is and scope of work and how big of a machine you can move.
I do some small res. clearing jobs now with my 8 ton machine and a skid steer with a mower, just dont price yourself out of the market, you can grind stumps with a mower just takes awhile, and anything that grinds is high maint.
hope this helps, i dont do much demo, so i wont try to offer advice.
god bless