Okay, this question has been asked many times and there’s a lot of content in the archives. However, the problem with forums is the content is difficult to find without spending a lot of time browsing the search results.
So, here’s my comment. I first bought a 425. I liked it so much, I wanted a larger version and purchased a used 1845, selling the 425 to my neighbor (he’s mowing the lawn with it as I write this response). I then missed the 425 and bought another used one. Since then, I’ve bought more property and I currently own an 1845, two 425’s, a Kubota L-3901, a Takeuchi T8 compact track loader, a Takeuchi TB-240 mini-excavator, and three zero turn mowers. I say that to say this, if I were forced to have only one machine, it would be a 425. That’s because it can handle almost any small job, along with larger mowing and bucket work. Its small size also comes in handy at times. Everything is a trade-off, between time, money, and speed. You only get to pick two.
For mowing a hillside with six-foot brush (is that diameter or height), you can forget about the 425 if we’re talking 6-inch diameter. That’s way too large for it. I have the brush cutter for the 425 but I switch to another machine if I’m cutting anything much over one inch in diameter. Also, you didn’t mention budget. I know, you want to spend the least possible but enough to get the job done. However, it’s more complicated than that. In many cases, it can be better to consider a used machine with more capability if new breaks the budget.
As to wheel motors on the 425, they probably still come with 14.2 cubic inch motors. I went from this size to 22.8 cubic inches. I still do not move around at full speed (about 5 mph) because it’s way too bumpy where I live and I’m getting old. My other 425 has 14.2 cubic inch motors. Although this tractor can go faster, the extra speed does not get used by me. Also, as you go up in size for wheel motors, the wheel motor mount must be modified (made larger). This is probably one reason why PT does not offer this as an option.
Since I don’t know your personal situation, it’s hard to make a strong recommendation. But I would recommend you carefully consider what the machine will be used for 80% of the time and buy a machine for that use. This could mean you need to buy used to save money. That is more hassle but sometimes it’s the only best option. For the other things you want to do, I would consider renting a machine optimized for that purpose. I’ve done this for years and it’s a good way to go.