I wouldnt think that you would want to clear 100 year old trees, just the small scrub stuff. A D6 will do that with ease and is really not that expensive when you look at the work it does. I assume that like most retired military, you will be looking at a second career and not want to spend 24-7 working on underbrushing. Getting too aggressive with tractor or other small equipment will cost you in repairs and wear and tear on the equipment much more than the dozer work. I know it is tough to lay out the money lump sum like that, but really that is the cheapest in the long run.
Get your tractor with FEL and add a backhoe if you really need it. I do a lot of stuff with my
B26 but it isnt a land clearing machine. Yesterday I cleaned up a blown down pine with it that was about 12" at the butt, dug up the stump, loaded up the roots and hauled away the tree, It took me about an hour on the one tree, so my comment is while a TLB can be used to remove trees, it isnt the most effective tool for the job.
With a 50 HP tractor with heavy duty bush hog, you can simply push over much of the scrub stuff with the FEL and chop it up with the bush hog. I cleared a lot of my place with a 45HP CUT and Bush Hog but pretty much did away with the hog in the process. Any thing that I could pushover with the tractor got chomped with the cutter. With the toothbar on the FEL, I scraped up the remains of larger ones into burn piles. My point is, you dont have to mow in reverse for scrub removal, if fact that is a good way to bend up your mower like I did backing into a briar thicketwith mine an hit an unseen object the pushed in the sheet metal that the blades the chewed up a bit. The tail end is not as strong as the front and hitting immovable objects with it will really bend it up, break off the tail wheels and even warp the whole frame. Best way is to lower your FEL with the bucket tilted up slightly so it will ride up on rocks etc and go really slow in thickets where you cant see what is there. For 4" diameter trees you may need to raise the bucket high to get some leverage to push them over but lower it down as the tree starts to go over.
Good Luck in what ever you decide to do.