Dann,
The 420 has a slightly larger engine than the 425 - the 425 is the entry model of the 400-series for 2000, while the 420 was the 400's older brother, and the 430's gas-powered twin. I think 22 horsepower, or around there. 48", 54" or 60" mid deck was available, also cat-0 3-point/PTO kit, hydraulic lift of the mid-mounted implements.
Chances are, they changed the mounts on the 425 and you'd have to modify the tractor or the attachment for it to fit.
Check over the 420 thoroughly if you decide to buy it, since even though they are workhorses, they can easily be abused and painted to look like new - 900 hours isn't an extreme amount of use but enough. Also make sure it was maintained; check the deck for repair welds and any other signs of the machine being overly abused.
Keep in mind that parts aren't cheap for these machines, but they are available within a couple days usually. The 420 with 900 hours will likely need repairs from time to time, so keep that in mind, both the cost and the down time.
The new 425 would run trouble free for exactly two years, and then you'll have to repair odds and ends as well. They are built that way. /w3tcompact/icons/wink.gif
It would really be your choice, to spend the money or not. If you decide to go with the 420, make sure it is in good mechanical order as it sits, and check into its history.
Best of luck,