I just got a used Kubota ZD326. I looked at new Scags, a used Gravely, and a ton of other stuff.
I am highly skeptical of the claim that a homeowner can't justify getting a diesel. If you have more than an acre or two, and you plan to be around for decades, a diesel seems like a good idea to me.
When I was shopping, I was told a gas engine is likely to die by 750 hours, and by 2,000, you should expect it to be scrap, while a diesel should go 3,000 before anything serious happens, and then it can be rebuilt. To me, 3,000 hours are probably 25 years.
Replacing a big gas engine by yourself costs $3,500+, so maybe $10,000+ over the course of 20 years, and it costs way more to have a mechanic do it. Unless your mechanic is a saint, expect to lose the use of your mower for a month or even three. What will you do about your grass while you wait? Diesel guys will still be mowing.
My understanding is that it's cheaper to rebuild a diesel than to replace a gas engine, and diesels are made to be rebuilt more than once. I'm no expert, so maybe I'm wrong, but this is what I have been told.
Gas engines have problems because of ethanol. I know about this firsthand. It is especially true where I live, with humid weather and all kinds of temperature fluctuations. Gas mowers burn more fuel and have to be refueled more often, which is really annoying. They are more complicated, so they have something diesel mowers never have: ignition problems.
Again, maybe I'm wrong, but I figured I could get at least 2,000 hours (call it 20 years) out of the used mower I just bought, with minimal repairs and easy maintenance. It may go a lot longer, and if it doesn't, I can have the engine rebuilt. I paid $5,500, and the best Home Depot gas mower, which is built to die after a few years of mowing 5 acres, costs around $3,000 more.
Additionally, Home Depot mowers are flimsy. My deck is 7 gauge. A Home Depot mower's deck is 10 gauge. That makes a difference if you want a mower that will last you 20 years. Home Depot and Cub Cadet expect you to be back in a few years to buy a new machine.
Speaking of new machines, I priced a Scag Turf Tiger II with a gas engine, and it was around $13,000 before tax. This is a really good mower. A Kubota would be somewhere around $18,000. Over 20 years, that's a difference of ~$300 per year. Seems worth it to me to get less refueling, no engine replacements, freedom from ethanol problems, and easier repairs and maintenance. And you can sell a 20-year-old Kubota. I'll bet there aren't many 2005 gas Scags on the market.
I was startled to learn how much money mowing your own lawn can save. Where I live, it could add up to over $100K in 20 years. That makes a commercial mower look like a gift.