4wd isn't just for mud...it's for traction all the time.... and on the few occasions where I take mine out of 4wd (to drive on pavement) I inevitably get get soon reminded to put it back in 4wd when working on the ranch...in dry conditions....because I'll lose traction, wonder why, remember that I'm not in 4wd, put it in 4wd and all is well.
My unscientific opinion is that 4wd is like adding about 15 to 20 hp to your tractor. I was renting a dirt buggy once and was asked what tractor I was using, with a comment by the county soil conservation person that typically the buggy was pulled by 75hp or better tractors. I told him about 50 hp but it was 4wd...he said I'd be OK...just kinda take it easy...he was right...it worked but I CLEARLY needed all traction possible.
4wd is all about getting the most from your tractor.
And, HST isn't just for loader work. On any place there is a huge amount of utility work where continuous speed control is extremely, extremely useful ...I actually consider it critical...and HST is the way to achieve this. Yes, HST shines in loader back and forth work, but it's value is continuous.
There are few folks, anymore, who keep their tractors in the field with dirt engaging implements 90% or more of the time. I'm talking hours a day, day after day. If you do, then HST is unnecessary for that application and even slightly less fuel efficient than a traditional geared tractor.
IMHO, it takes 4 times longer to do the same FEL dirt work job with 2wd gear versus 4wd HST. I've tried using only 2wd just to see what happens. For me, in 2wd, I have to seek to take bites of a pile several times to get a heaping bucket full of material, backing up and going forward and playing with curl and lift, etc... with 4wd, I get a full bucket about 85% of the time, first time.
You are going to use the tractor on 500 acres.... sure, people have handled that kind of acreage for years using everything from horseback on up. Commuting between DFW and Stonewall to manage the property is time consuming enough without having a tractor that is less than fully capable of not only starting but doing all the tasks at hand when you get there without you having to fight the capabilities of the tractor while you are doing it.
So, my advice, is you not go short on tractor capability to handle the 500 acres. The threads of TBN are replete with stories of folks who purchased less then moved up to larger tractors/4wd/more equipment/etc and few, extremely few, are dealing with 500 acres, many are far, far less.