in terms of budget, around 5.5k & below will get you a consumer grade box store mower. of course depending on size. for heaver duty or commercial, plan on spending 7k on up. in general, 54"-60" are the std sizes
"My Son has the I-mark mowing 2 acres": Ricn
think it's the eXmark mower. plenty of good quality out there, Ferris another one. the OP will have to look at 8k plus for that grade mower.
commercial mowers not only have a more robust frame construction, but heavier duty wheel motors & hyd pumps. (Parker & Hydro gear are the ones to look for on those higher end items) ... in terms of mower engines, i lean toward Kawi, regards