Hey, have you made the decision yet? Pretty good tractors. Have heard of PTO not engaging all the way after running PTO for several hours (ie. stop, go to lunch, come back and won't engage) but that was on a T4.100. Seems there were some various issues with the T4.100-120's but never heard any real complaints from the 75. With only 400 hours you should be fine. NH was running some really good deals on T4.75's a few months ago which put a new tractor very close in price to a late model used, so maybe the seller realized this. I was selling T4.75's in the low, low $40k's with all available incentives when I was a NH branch manager. NH was clearing them out to make room for the 2019 Workmaster 55-75 Cab series. Been a while, but I think T4/Powerstar 75's are now selling new around the $50K mark.
I sold 3 or 4 T4.75 and/or Powerstar 75's in a year's time or so. The owners liked them. Only complaint from one was that it was a little light on HP for running a 4x5 silage baler on hills. But he absolutely loved it for feeding round bales between pastures (ie. transport between farms) and tedding hay. I had another customer run a metal post into the fuel tank under the tractor during first week of ownership (just a freak accident). Wasn't the tractor's fault, but he thought there should've been more shielding under there. Just a word of caution.
Let us know how it worked out!