There's a lot of great replies here ... getting me thinking in lots of directions.
The best thing I have to compare to this experience is cars ... I've been into cars for a while and like working on them so I can compare some lessons learned there to here.
1. I'm certainly of the belief that I prefer a more powerful engine vs a less powerful one. So whatever motor I get I would want it to be the equivalent of a v8 powered SUV - pretty much more than you need, which means less stress on the motor.
Then what you want to look for is the largest displacement engine with the lowest engine RPM required to achieve 540 PTO RPM.
2. I don't know anything about Tractor motors but when I look at vehicles I look at the motor and try to find bulletproof ones. Are tractor motors made by certain manufacturers and shared amongst tractor brands? Are some motors known for their reliability over others?
Not really. Branson tractors are powered by what amounts to a Cummins if that helps.
3. I have looked at some used one .. the hours I've seen seem to be 1500 hours or less ... how many is too many? Also are there "service" intervals for tractors wherby a certain "hour mark" they pretty much require, new this, new that, etc?
Maintanince is going to be the issue with any used tractor, just like a car. Typically I wouldn't be afraid of anything with less than 5k hours on it. Service intervals are throughout the life of the tractor and you shouldn't shy away from a well maintained tractor just because it's coming up on a service. You'll at least know that one was done properly.
4. I think maybe my eyes are bigger than my wallet right now, and the more I think the more maybe a backhoe isn't a critical accessory I need right now. Are there any critical "must have" accessories when buying? For example is a bucket loader a must have?
A FEL(Front End Loader) is the most critical implement to have for most of us. It not only allows us to move material in the bucket, but is a platform for adding forks, snow blades, and grapples. You can do a lot with just a FEL, shredder, and box blade.
5. With tractors, is rust a concern? Do some manufacturers use better metal than other? Or maybe older built machines have better metal than new?
Tractors are not made from thin hydroformed steel. Rust only becomes a serious issue if the tractor was used to spread caustic substances(salt/fertilizer), and was not properly cleaned afterwards.
6. When it comes to attachments and parts are tractors universal? Can I mount any type of tractor backhoe to any tractor?, what about other accessories?
No, backhoes are proprietary, and typically require a subframe. There are 3pt versions, but they are not recommended as they put undue stress on your 3pt assembly and have been known to rip the top link mount right out of the rear end housing. Any implement that connects to the tractor using the rear PTO, 3pt hitch, or drawbar will work across any brand. Those each have their own requirements, PTO HP (min and max), 3pt category(1, 2, 3), and max towing weight of your tractor.
7. What usually breaks on tractors? Are they hard to work on or fix? Are some tractor parts more "universal" than others, or easier to find than others?
A tractor, at the end of the day, is a diesel vehicle with a hydraulic system. Most breakages are either hydraulic or electrical. I'm unsure how to best answer your "easier to fix" question.
8. What tractors do you think have the best communities around them? One of the most helpful things for me is when I have a problem, to find forums and others that have had much experience with the same machines.
By shear volume, Kubota, Case, and John Deere have the largest communities. Case doesn't really play in the compact realm much.