I could NOT mow "my" lawn with a 5' mower with that few HP. I think you will be very disapointed if you buy that combo.. My grass is thick, and if i left it two weeks, it takes a LOT of hp to mow it, at any speed at all.
One thing i found out when I TRIED IT FOR MYSELF is, that a 3 bladed finish mower takes "more power" to mow with than a rotary cutter. (brush hog) You are turning a lot more berrings, belts, and that adds up to quite a bit friction. Then there's the problem with the finish mower cutting and recutting the same grass before it leaves the mower!
Also keep in mind, that a "rear discharge" mower will take less power to mow with than a "side discharge". (gets rid of the grass easier, faster)