I have severe RA under constant treatment by a specialist so I can empathise with you. Brand is less important than function in my book. Definately go with HST transmission. And go with a more modern design than either the NH TC30 or the JD 790 because those don't have the ergonomic design that newer machines have. Look for a tractor that 'fits' you, some have fender mounted front loader controls and that may be easier for you to reach/manipulate. Others have loader controls that are mounted on the loader and those may be a harder reach. Also look for other little things like the New Holland "turtle/rabbit" control that is on the loader lever of some of their models, other brands have a similar feature, but it gives you greater control of the machine and is as simple as pushing a button with your thumb.
There are plenty of bargain tractors out there, but in your case I would strongly avoid them. Bargain machines, while they may be strong and reliable, typically do not have the features that YOU will find useful given your reduced right arm function.
Adjustable lower links are a help, but I'd spend the extra $200 and buy an aftermarket set of "Pat's Easy Change" 3pt connectors if you are going to be swapping a lot of implements and one of them is a post hole digger. If you are not using a post hole digger, then get one of the other brands of Easy/Quick Hitch systems (available from most any dealer).
Look for a suspension seat and an adjustable/tilt steering wheel. The suspension seat will take some shock out of your body on bumps (tractors do NOT have shock absorbers). Some tractors have steering wheels that you can tilt and/or adjust in multiple ways. That little thing may be the difference between you being able to control the tractor with one arm or not.
I will say that both New Holland and John Deere have some of the best ergonomic designs of any brand I know. Both brands impress me greatly for some of the design details and features they include that are simply not on other machines. Kubota outsells both JD and New Holland combined, and I own a Kubota, but I have to tell you that in your case I'd seriously look at the New Holland TC29 or TC33 and the JD3120 and JD3320 machines. Kubota would have
B7800,
B3030 and
L2800,
L3130 and
L3400 machines in the same ranges as you are looking at. The
B7800/
L2800/
L3400 machines are basic and I would rule them out given your circumstances. The
B3030 and
L3130 are deluxe machines but I beleive both of them are lacking in ergonomic convenience when compared to the New Holland TC29/TC33 or the JD3120/3320 machines.