OK, Steve, I don't know of anyone who has really reliable statistics on the most reliable tractors and best parts availability and price. There are many really good, long lasting tractor brands, but any of them can have a problem now and then. I know Deere has a very good reputation for parts availability, but as to whether your dealer would have what you need in stock; that varies. And as for overall best price on parts, I have no idea. I'm old enough that everything seems high priced to me now-a-days. I afraid your question is kind of like asking what brand car you should buy, considering the same criteria. You'll get lots of different opinions.
It is true that, personally, I'm a little partial to Kubotas and have more confidence in them and the Kubota dealers I know. But that's more of a personal feeling; no proof or hard evidence. I helped a cousin shop for tractors and he bought a New Holland; nice tractor. I have one friend who won't buy anything but Deere; he has 3 of them. And if I were going out to buy a new tractor today, I'd probably go look at some brands other than Kubota, and if I found one I liked that had all the features I wanted, at a price I could afford, from a dealer I thought would provide any service I needed in the future, that's the one I'd buy.
Personally, I'd probably only consider Kubota, Deere, New Holland, Case, White, and Massey Ferguson because I've driven those at one time or another, I know some dealers, and they've been around awhile. Now we have some "relative" newcomers to the market; Mahindra, Zetor, and Kioti seem to be coming on strong and I suspect may become major names someday. The problem with any "new" product is that it may be great, or it may not. You have to decide whether resale value is important to you or not. And are you willing to take a chance? I've met some folks who bought Belarus tractors for about half the price of a comparable Deere, and every one of them I've met has regretted that decision. But we'll never know unless someone tries them first. Now we have a number of Chinese tractors, and your guess is as good as mine as to whether they'll be a good investment over the long haul. Again we'll never know unless someone buys them.
Whatever you buy, I'd strongly recommend that you include the workshop or service manual and the parts manual to start with. And buy it from a dealer you have confidence in.