When I buy a piece of equipment, I look for the gauge of metal and the hp tolerance of the gearbox. I like to have the hp of the tractor be the minimum for the gear box, they tend to last longer. I have had light duty mowers, and that is just what they are. Mowing any size brush at all and the sides will be beaten out in no time. The last one I bought, before I even used it I put 3/16 under the deck all around and 3/8 along both sides and the back. I have used this one for over 6 yrs in some tough going and it still looks great. I have been through several sets of slipclutches, but other than that, no problems.
One other thing that has helped considerably is that I use blades that are tipped down on the cutting side. With the back of the blade higher than the front, if you hit a big rock or whatever the back of the blade will hit the deck before the cutting edge. I have had straight blades rip the deck like a can opener.