Seeing mention of turbo diesels, I think someone now offers a TD 33+hp CAT on one of their machines. That's gotta be nice. My old Ransomes had a 3cyl. 23hp Mitsubishi which did well.
I'd be okay with a 26hp B&S commercial series myself. I used B&S and Tecumseh, Onan and Kohler and the only one I didn't care for was Tec. because they burned oil nearly as quickly as gasoline. Back then, B&S was the least expensive and they really were good engines. Honda came along and blew everything else away with true bearings, which allowed for easier starting and longer life... But MAN were they expensive(still are).
I still say any commercial grade will do for under 15 acres imo, but I have preferences too.