DX cameras can take some great photos.. if your shooting sports or wild life the extra crop you get with a DX is nice.. my 600mm lens actually gets me to about 900mm on the crop sensor.. something like the d7200 or d500 would be the way to go for a crop sensor if you want good quality camera with lots of features.. even something like the D5300 will take excellent photos with a nice lens on it but there is not a built in focus motor or anything liek the other 2 i mentioned has... full frame or DX lenses work great on a DX camera..
if your not into sports or wildlife and you dont mind spending the extra money i would say go with the full frame.. if using a DX lens on a FX camera you will want to switch to DX mode in the camera body.. you loose some megapixels when you do that so its best to get full frame lenses.
I bought a new camera the other day. I got a D750. I fiddled with a D500. Oh, I really wanted that camera for the FPS. It was a toss up nearly. I thought of buying it, using it then returning it within 30 days if I changed my mind. If I knew my d600 was repairable I may have gotten the D500, but I still don't know that. I figure I have 30 days to find out and return the d750 for a d500 if I really want to. I really need the FX low light for astrophotography - cuts the exposures in half vs DX. Plus the DX would technically take my telescope to 7,500mm LOL
well that D750 is a nice camera, if i were going full frame that is what i would get... hopefully you can get your other camera fixed...
the high iso comparison between the d500 and the d810 looked to be about the same to me, the photos looked identical and appeared to have about the same amount of noise , the highest they went in the test was 12,800 iso though. i have also saw a few photos taken at around 50k iso with the d500 and the photos were still usable.. noisy but not crazy noisy, just a bit more noisy than i would want but your talking in the 50k iso range.... here is a link to that comparison.. it sounds like you would really like to have the d500 and from what every thing i have seen or heard that d500 sounds like its almost as good as a full frame in low light..
The Nikon D500 - Reigniting The DX / FX Debate And A Few High ISO Photos