Arkydog,
I've seen a variety of industrial as well as "homemade" solutions for vehicles on slopes.
Here are some, if you can picture what I'm describing.
1. Set rear wheels extra wide
2. Similar to #1, but use an extra wheel hub between axle and wheel (2 hubs on one side, one or both with tires). Sort of like "duallies" on one side.
3. Extended axle on one side. This one is tricky due to balancing and reinforcement. The one I saw used an axle from a larger tractor (longer) on one side. Not sure how it was connected.
4. Using "outrigger" with a wheel on the end. This could be hydraulic to adjust for position. Bracing would be important.
If you must mow across a slope, it is easier to always mow from one side (leaning either left or right) and modifying only one side.
What else do you (in a perfect world) want to do with the tractor? Most of the above would still allow you to do other things with the tractor.
The specialized unit you posted a link to is very interesting, but seems more for a single purpose, like a commercial situation. It also looks like it may slide on an uneven surface, something to watch out for.
Pretty cool, but chances are good that once you get a tractor, you'll find all kinds of new uses for it.
Again, it depends upon how you want to use it and which option you are most comfortable with. Since you are new to tractors, you may not feel comfortable making your own modifications.
If not, don't do it. Most important thing here, is don't get hurt. A roll can and does kill, so be sure.
Good luck, and let us know how you decide to go.
-JC
PS- Welcome to TBN!