I buy the cheapest, highest horsepower, B&S engine, non-self-propelled model I can find in my price range. I never change the oil in them, the mower body always wears out before the engine does and I just add oil as necessary and sharpen the blade once or twice a year.
I have a 1972 Cub Cadet 149 that still runs but is to big for my yard, it was my dad's and he spent many hours mowing our churches lawn and cemetery every weekend on it. I am going to be restoring it in the very near future.