hills, hills, and more hills. and emergency brake not working, and frequent on/off.
a hydrostatic transmission, i would most likely stay away from. it doesn't matter if you are in gear or out of gear. they will roll on you no matter what, and only way to stop them is with the emergency brake when tractor is off. since you noted pretty much a "pull tractor" and not a multi task tractor that a weekend warrior might have. i would more look at a standard manual transmission. this will let ya set a gear and let the engine brake for ya some going down hills, and leting you shut off engine, and hold some what, before it finally rolls.
i doubt you will need a creeper package from sounds of it, but more likely a generic manual transmission, and if it offers a few more gears say (hi,med,low) vs hi/lo, get the hi,med,low package. more likely difference between 8 gears to 12 gears or some such.
if ya running hills, consider "spacers" for rear tires. i would most likely stay away from "duals" (2 tires per side on rear), or industrial R4 tires. they give to much floating, and would more likely let rear end slide away from you. and in that the standard agriculture R1 tires more likely better fit for ya.
your wet clutches and wet brakes more likely to last longer, than dry brakes. (clutches and brakes set in oil inside of everything)
your elevation could be an issue. runs good at the bottom, but rough at the top or vice vs. a turbo charger could possibly help with this. but at same time, if you are having employee's run machine. bad habits, of not dealing with turbo charger correctly = short life of turbo charger. turbo chargers need some "cool down time" before ya shut tractor down. if tractor run's poorly, you may need to see if manufacture can adjust the computer chip for fuel injection, to deal with elevation, and getting correct air/fuel mixture, regardless of turbo charger or not.
if you do not run any city,township, county, state, roads and just your tree farm, you might consider chains for tires. for extra grip year round.
some manufactures do not recommend filling rear and/or front tires. (for rear tires, it is normally when a backhoe is placed on it), and filling rears with fluid is normally fine in all other means and for the fronts, the front is just not heavy duty enough, normally the operators manual will give this sort of info. and the front tires if they can be filled or not with a fluid.
front weight rack mount may be an additional cost, for suit case weights, along with additional cost for rear tire rim weights.
if ya goto most major manufactur websties, you can "build your tractor" through there website, they normally offer different size rims and tires. you might be better off going directly through a local dealer, if you want smaller size tires. to help lower COG (center of gravity) on the tractor. with 4wd / MFWD or like, you need to keep correct ratio between tire diameters, so ya don't overload things and bind up the 4wd stuff.
there are some companies that are purely dedicated to modify standard tractors from major manufactures, for certain conditions, say orchard farmers, or vineyards, to those folks that work the ugly slopes. most of what i have seen from research in the past, was the final drive (what comes off transmission / pto hosing, and extends out to the rims. and completely replacing what comes from manufacture for there setup. i came across more "combine" specialized companies than i did with regular modified tractors. (combines needing certain angle and if to steep, your crop is not harvested but blown out the back)
i can not remember it, john deer, and kubota does not have it, but i want to say mahindra does, it is something to do with PTO or transmission. it was a function, that week end warriors would most likely never really use. but it may serve you. but *arghs* it is right there on tip of my tongue, something to do with reverse something is what i want to say.
one other issue that came up from research long ago. was oil pan on engines. some engines could not handle angles very well, due to all the oil would run to one side vs the other side. and it was how the oil pickups going to the oil pump were and shape of the oil pan.
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take above with a grain of salt, i would be more arm chair bozo, vs real life experience than what others have had for your conditions. and i could be making more of a big deal, than what it should be.