You can, but wheel motors themselves are not inexpensive.
What you end up chasing with slope mowing up/down is raw, total power output per unit time, aka horsepower. So going for larger wheel displacement motors will get you better hill climbing power, but at a slower rate of speed. The larger wheel motors will commit you to lower speeds always, in all tasks.
The 1445 is almost double the HP of the 425, some of which is used by the larger mower.
Could you get the work done in a 425? I think probably if your slope isn't over 20 degrees or so. (The upper maximum on the engine was around 22 degrees, if I recall correctly, due to oil starvation.) Would you need to make accommodations? Almost certainly; slower, different mowing patterns, etc.
The 45HP Deutz is happy to do a lot of work at just above idle, whereas the gasoline engine in the 425 needs to be closer to wide open throttle always for optimal air cooling. The Deutz ends up being quite fuel efficient for me as most of my chores and tasks are done closer to 1/4 throttle. (Mowing, rototilling, chipping, and trenching being the exceptions)
At the end of the day, I think it is as
@woodlandfarms suggested about buying the HP that you can afford.
I looked at the 425, and Power-Trac talked me out of it due to my steeper slopes. I am very happy with the 1445, and use it pretty much daily for something, and being able to lift 1800lbs comes in handy far more often than I thought it would.
All the best,
Peter