Buying used is fine, as long as you get a good enough deal to justify it. You've already brought up great points regarding better financing & warranty. I used the warranty early-on with my Kioti, which would have otherwise cost $1500-2000, just for the one repair. Never used the warranty on my LS machines, had two in a year, put about 200 hrs between them. Haven't used the warranty on my Massey yet, 11 months and 370 hrs. But then, my buddies Kubota had to be split at just over 1 yr old to repair a small part hidden inside, tractor was out of service for two weeks and probably would've cost $3000+.
Can you do the work yourself if something breaks?
Have you found any used tractors with remaining warranty?
Regarding your brand shopping, both the Massey 17xx series, and the Kioti NX series are good, but I personally feel the Massey is better. My local dealer sells both, and feels the Massey is a better machine, but that the Kioti is a better value.
Don't forget, Massey (AGCO) offers a huge discount for their "Equine Savings Program" ... So if you have anything to do with horses, tell them and you should qualify. In my experience, there's nothing you'll need to do to prove anything about your horse operation. Check into it...
Good luck!