Speaking strictly from my experiences with my PT425 for about 20 years, it will not mow the slopes you are describing to your satisfaction. A couple reasons. 1, the KOHLER engine in my unit is limited to 25 degrees due to the lubrication system. 2. as the oil heats up from mowing, after several hours, the machine will lose power making it harder to push a running mower up a slope.
Do you know the degree of slope of your properties? That is important.
Next thing to look at would be asking PowerTrac what engine they are currently putting in the PT425. They used to use Kohler, then they switched to Robbin. Those had some issues with colds weather starting, but otherwise I think there are some very satisfied Robbin powered owners. Rumors have it PowerTrac has switched back to Kohler, so ask them to be certain.
As for the diesel powered PowerTracs, as far as I know none of them are air OR liquid cooled. The Deutz engines are oil cooled.
Back to gas engines. I've never had the Kohler fail to start down to -5F (I give it a short squirt of starting fluid if it's under 10F). I have never used gasoline stabilizer. I have only used 87 octane 10% ethanol. The unit has sat for months sometimes with no use. It's stored in un-heated sheds and garages. Always starts. For that matter, I've used 87 octane 10% ethanol untreated in ALL of my small engines and automobile engines since the mid 80s. Never a fuel issue. Ever. Follow your manufacturer's recommended fuel procedures and you'll be fine.
Fuel consumption on my PT425 with the Kohler is just a tad over 1 gallon per hour. Put 4.5 gallons in the tank, run it till lunch, take a break, refill, run till supper. No issues.
Mowing is the most taxing activity on the unit. Similar with brush cutting. FEL work is not very hard on the unit at all. Neither is snow remove, fork work, etc... only mowing causes the heat build up over several hours.
It sounds like your main concerns are mowing, given you comments on your JD unit and lawn damage.
I live on one acre of grass. The PT425 mows it every week. Very flat lawn. Takes about 25 minutes. I like the results. It's not a show lawn, but it's still the nicest in the neighborhood (until the rain stops :laughing
. We also have 20 acres of hilly property 9 miles away. I use the PT425 to brush hog trails and meadows, remove firewood, etc... I have some steep slopes and have no problems, but I'm not mowing large areas on those slopes, only two-pass-wide trails.
Another thing you should be aware of is conventional tractors are meant for pulling, and they excel at ground engaging implements like pulling dirt plows, box blades, etc... the PT units are better loaders. They're built for it. In my opinion, the PT units are better mowers and brush cutters in tight spaces, lawns, trails, etc.. where as a conventional tractor would be great in large open spaces.
Having the implements out front where you can see them really makes a difference. Brush is cut down before you run over it and you don't have to strain your neck looking behind you.
See if you can find someone close to you that might demo a unit for you, or make the trip to Tazewell. I'm guessing you'll gravitate towards the 1430 sized units.
Good luck in your search.