The opinions on engines vary all over the place. My Husky uses a B&S 24HP 2 cylinder vertical shaft engine, and quite frankly, I don't recall ever seeing a horizontal shaft engine in a riding mower....perhaps in some high end zero turn, costing thousands more.
Engines have been improved substantially in recent years, and with proper maintenance, virtually any of them should last for many years. I have a neighbor who has a 20 yr. old Husky, and it is still going strong.
BTW, I hear ya on the roof...we had ours replaced this past April...$6k.