John Deere and Kubota are the only modern garden tractors I know of that can be had in Diesel. Diesels last forever and are very powerful. But, you really don't need it for cutting grass. Besides, they are so very very loud. I prefer a good liquid cooled gaser for mowing and yard work. So much quieter to be around.
I would suggest going used. Don't know the bota models. But in deere look for a 420 or 455 for diesel, or a 317, 318, 425, 445, 345 for gas. The 445 is nice because it is fuel injected, which should mean less problems with this crappy ethanol gasoline. Newer JD models are the X500 and X700 series.
I have a 425 all-wheel-steer and I just love it. I bought it new in '94. I've spent only $75 on repairs. Extremely reliable. It's a pleasure to mow with and great at plowing snow when equipped with chains and weights.
Any way you go about it, a top quality garden tractor will cost you a few bucks.....and worth every penny.