Honestly, I feel any tractor will fall apart around the engine, long before you will need to think about rebuilding it.
Quite frankly, in a garden tractor, you will never reach a point where you will need to rebuild a diesel engine if you have one. The hydraulic pumps on these small machines, while good, are not the kind of pumps you would find on a $250,000 excavator. At around 2,500 hours, it comes time to think about rebuilding your hydraulic pump(s) and your trans-axle. It will take years and years to reach this point. At this point, myself, I would buy a new tractor. I don't believe in throwing money into old equipment. You can forever try to keep old equipment running, or just make a payment and it works every single time you go outside to use it. Either way, regardless of what some might say, you WILL spend the money. Pay now or pay later.
I recently seen a X495 I think it was, with the Yanmar diesel. It was at my local dealership and had just been traded. It had close to 5,000 hours on it. The engine was still running, sounded great, but the transaxle was about done and it was on it's 2nd hydraulic system, ready for it's 3rd. The guy who used it started a commercial landscape/maintenance company. If this machine was not mowing, it was running a big rotary broom through the front PTO. It had a hard life and worked a TON of hours. To this date I have never seen a garden tractor with so many hours on it. Needless to say, Besides the water pump and the alternator, the mechanic was pretty sure it was all original. It was sent straight to auction and sold for next to nothing, really isn't worth anything at that point.
Like someone else said, use the tractor more, worry less. I just bought a 2012 X740 in February, I have 50 hours on it now. I don't even have my 54'' Quick Hitch blade yet, when I do, my hours will REALLY go up. I don't sweat it. I bought the machine to use it to it's fullest of capabilities. Every hour on the hour meter is an hour NOT on my back and legs. I plan to put about 100 hours on the machine per year. Even at this rate, I will get 20 years out of it, 16 of which will be payment free. I worry more about the hydraulics and trans-axle than the engine. Actually, the Yanmar under my hood is the last worry on my list.