Have you bought yet?
I've owned 3 JDs mainly because the JD dealer was so much closer than any Kubota dealer. Now, there are 3 Kubota dealers fairly nearby (such that they won't charge that $450 to transport mentioned); the JD dealer charged $120.
A 1 acre veggie garden is huge. I had a 20 row garden about 50 ft long rows, and grew so much that I gave away a lot.
1) How much horsepower do I need? A relative has told me I shouldn't go any smaller than 40hp. I don't see myself doing much in the way of dirt work, is 40hp more than I need?
For my 8.5 acres and bush hog, wood chipping and snow movement work and a tad of dirt movement with a box blade, an 18.5 hp machine like the first one I had is completely adequate. Cannot get an 18.5 hp in a Kubota B nor JD 2 series (need the extra height on our hilly property); so, 2 of the JDs have been 24.5 hp, and Kubota B2601 is 25.5.
My neighbor on 5 acres has a 45 hp JD. He bush hogs and maintains a gravel driveway and moves snow, all tasks that an 18.5 hp diesel would do fine. His big tractor's weight probably is better to hold it in place better on his gravel driveway. Mine is only about 400 ft or so, relatively flat and paved.
Only time I've ever needed more hp is when trying to remove stumps with the FEL. With more hp, I'd probably end up bending the FEL though.
2) I've been encouraged to primarily shop for John Deere or Kubota. I see a lot of LS, Mahindra, Yanmar, Kioti, etc. sitting in my neighbors fields. I don't mind paying more when the quality is superior, but for a consumer like me am I going to realize the benefits of the big two brands there? None of the engines (up to 25.5 hp) have the Tier 4 stuff on the exhaust, and Kubota does a MUCH better job of controlling emissions such that their idle speeds are not elevated like the current JD's. The B uses 30% less fuel than the JDs (JDs 50% more than the B, all of them). From my experience since January on the B, it is many many times better than JDs in so many areas. The main thing I don't like about the Kubota is the reverse treadle pedal. It has gotten easier to use now with near 70 hours on it. Also, there are 2 engine oil drain plugs on the Kubota. I went years ago using an oil extractor on everything. Cannot use it on one side of the Kubota engine.
One thing that happened on my first JD was the Ujoint off the engine blew apart at 9 years and 660 hours. The Kubota B has no such Ujoints. The HST is carefully lined up with the engine. Little sleeves combine the shaft with the engine and HST at each end.
Have no experience with the other tractor makes, but the Isuzu generator driver (about 20 hp, 12.5 kw generator) is a real gem. One of those mentioned uses Isuzu engines, maybe LS?
3) Any special considerations when purchasing used over new? I haven't decided which way to go here, will probably depend on "how much tractor" I end up deciding that I need.
With today's prices, you can get a new one for very little more than a used one.
4) HST vs gear drive? Definitely HST for your situation.