I have a John Deere Sabre model 1438 HS with a 14.5 B&S engine on a 38-inch
deck used on 1.5 acres. Now with the bigger 42" decks and hills in your
case I would go with no less than a 20hp plus motor.

Now if you get a bag unit you *MUST* make sure to get high lift bagger
blades as well. My bag unit works just fine as long as the grass is not to
wet or thick. Now this is what I do to really get good results with my
bagger.

I try to get my lawn cut before noon without using the bagger, and I just
let the clippings set out in the sun all afternoon or even overnight which
both dries and shrinks the clippings. Then I go over it with my bagger and
WOW!! Does it ever clean up good.

Now this little trick also works pretty well when the grass is too wet or to
high to use the bagger. Now with really heavy and thick grass I first will
mow it nice and slow, and with the chute tired open if there are no kids,
pets, windows or any other concerns. Then I go over it again in about 2
hours, but at high speed to stir the clippings for bettery drying. Then
when I am ready to use the bagger I raise my cutting deck usually 1/2 to
allow maximum suction and air flow, and then I just go over it really slow.

Good Luck!!!