koop,
If you're only cutting a few acres any Scag will probably out last you. Vertical vs. horizontal engine doesn't really make any difference. Belt drive vs. shaft drive is a matter of opinion. Belts are easy to change, they will dry rot before they wear out unless you're using the machine for commercial use. Liquid cooled engine is a nice feature because it extends engine life. Air cooled is nice because there are fewer componenets to fail...(radiator, water pump, hoses etc.). An air cooled Kawasaki engine used for residential use will most likely out last you.
Jeff