The three mowers you are looking at are probably equal in quality. All are light duty.
The Husqvarna is equipped with ball bearing wheels. This would probably be easier to push than the troybilt and craftsman. The Kohler engine is actually a Chinese design that has some good features including a cast iron sleeve and double ball bearings on the crankshaft. On the negative side, the engine is designed with several layers of trim that make access the the engine more difficult and perhaps heavier. I worked on several of these for "will not start" conditions related to last year's fuel. When testing, I found the engine seemed to be under powered for a 21" Ariens mower. Consider this carefully, as the engine has only 149CC - a very small engine. None of the kohler dealers in my area carried any parts for the push mower engine.

The other mowers have the regular plastic wheels without the ball bearings. This will probably require more effort to push. The engine on both is the 190cc Briggs quantum series with ready start. This engine has good power and is easy to start. Parts like carburetor kits are usually stocked at dealers and the parts are cheap. On the negative side, the automatic choke sometimes gets confused - resulting in a will not start condition. The condition goes away if the mower sits for a while - or the home owner can manually open the choke to start (provided he knows how to do this).

I guess the Craftsman is now a "parts" mower.
If a severe oil leak, I'd say something is cracked , more than just a seal...
"Lite-duty" mower ?
On my turf, only the moons surface or NYC streets are rougher ! I need "heavy duty".
Maybe an Ariens with caster wheels.
The exercise is important...not that much "big E" with a rider..
IMO...