To further explain something you should consider that the engine is responsible for only two primary functions, driving the wheels (via hydraulic pumps) and rotating the blades (via PTO clutch). Resistance on either system will create a situation where the power, more specifically the torque of the engine will matter. More powerful engine can certainly handle more adverse conditions such as driving up hills (still limited by pumps as well), but also mowing through long, thick or wet grass. So, with that being said, keeping the blades sharp at all times, slowing down mowing speed to handle clippings when you let it go a little long between cuts and always running at full throttle will help keep the cut good. So will you notice a difference between the various engines you spoke of, not likely unless you are mowing at full speed and trying to cut off more than 1/3 of the grass in wet conditions. Of course not all mowers are created equal, hence why they range in price so much, but what you want to spend provides you with a set group of specifications. Ground speed, blade tip speed, engine make/model, hydro model, etc.. i genuinely think that there are no "bad" zero turns out there when price is the determining factor, but there are definitely ones that are better than others when comparing side by side; just depends on what is important to you.