Looking for a full frame night sky camera

Hi All,
I'm a new member and haven't yet had time to view many posts, so please forgive me if this has been covered.
I currently shoot a D300 and a D7000, using the Nikkor 10.5 f2.8 and Sigma's 18-35 f1.8 for night sky. I've been a Nikon fan since the 70's, when I got my first - an F2. I'm getting very interested in night sky photography and feel I need to jump to full frame as a result. I'm looking for opinions as to which of the reasonably (?) priced FX models I should go for. I've heard good things about the D800e and thought I'd jump to the D810, knowing that in some aspects it's a noticeable jump forward. But I have also heard that the 810 has thermal noise issues compared to the 800e.
As I'm satisfied with DX for most types of shooting and am interested in getting into FX for night sky only, I don't need a lot of bells and whistles. I'm also an old timer and was taught to depend on manual control, so my main concern here is low long exposure and high ISO noise. I'm also willing to get a used camera, which opens up the possibility of the D700 or the D610.
So, which FX?
Any and all opinions welcomed, and thank you.

I have a D700, but I've never tried night sky shots. So I'm afraid I'd just be guessing as to how well the D700 would work for that type of shots.
Sorry. But someone should be by that will know!

If you're on a budget, find a D610 at the best possible price (they seem to be in the $1396 rang now which is amazing) and go with that. Great low light performance, and if all you're looking for is night sky then that will do you wonderfully. But, if you've got the money to spend (which I suspect you do since you mentioned the D810) you might be happier down the road with the D750. I never wanted the articulating screen on the back, but I've used it a dozen times in the month I've had the camera, and it would be very helpful if you're setting up a low tripod and want to frame the shot. It's also click better on high ISO noise - but it's $900 more that the D610 at this time, and whether or not it's $900 more camera depends on the photographer.