I am not the most versed in the various tractors, so take everything I say with a grain of salt. 300 hours is nothing for 5 years of use. The tractor is still new (depending on how you've used it, which only you know). If it does everything you need it to minus the functionality of a few implements, I'd hang onto it. If you need new implements, when buying local they're generally cheaper when purchased with a tractor, so I can understand the draw towards buying an entirely new machine. Have you checked places like (Temporarily blocked due to reports of company closure)? They will ship free to you.
If you have what you feel is good equipment, keep in mind that good equipment generally makes it to 5000 hours without major problems (beyond those caused by the operator).
I can also tell you, no matter how big you buy, more HP always sounds handy. I just got a 4035HST that has three ranges, I'm in WV, you're in TN, so we're in the same mountain range and likely have similar slopes, I too must cut and work in low range on my hills, and I can't even get low range to hold full speed on hills even without the PTO running. I have my rear tires ballasted, and just the bare tractor with nothing running, attached, or in tow it still wants to crawl up my hill; perhaps I should have opted for the 5035HST, or maybe a 60HP of some sort. Problem with that line of thinking is as the HP increases, so does the overall mass of the tractor as well as the mass of the implements we'll try to tote around. I don't think any of the HSTs could cut my hill in better than low range when taking all size and weight variables into account.
Honestly, I think these machines are all intended to do 99% of their work in low range, and other ranges are simply for moving the tractor around when not under load of the PTO and weight of the implements. I think the lack of power to work massive hillsides at high speeds is as much of a safety feature factored into the design of tractors as much as ROPS and seatbelts. It's not a feature we all like or appreciate, but it's one that keeps us shiny side up/rubber side down even better than ROPS. Working a hillside in mid or high range is asking for disaster if ya find a groundhog hole in a bad spot, you need to be going slow enough there is no "bounce" during such occurrences (I am not saying that you cannot roll over without a bounce, but a bounce on a hillside is almost always a sure slide or can be good to lift a wheel or two off the ground). Running a tractor on a hillside isn't much different from 4x4 rock crawling, the key word is crawl...
