I would advise you to try and go somewhere where you can handle the cameras you are interested in, as this seems important to you.
Only you can say what your needs are, FX or DX. Either will fulfil almost all situations, but will have slight benefits over each other in certain areas. Ultimate image quality (whether you can see it is another thing
), better low light performance for FX. For DX you get AF coverage over more of the scene, lenses appearing to magnify and get closer to the subject because of the 'cropped' sensor. Frames per second will depend on the camera, as will the buffer depth.
For me, I take pics of different things, and sport has always been part of that. So I always wanted the fastest camera, AF and/or Fps, that I could afford. Having started with DX, and found it more than good enough for me, so I was never drawn to FX, and my Nikon journey has been D70 > D200 > D300 > D300S > D500. Most have been an improvement over the previous camera, otherwise what would have been the point in changing,
but the D300S > D500 is the jump that improved on the previous model in almost every way. It is the ultimate crop sensor DSLR, imho, and will probably not be bettered.
Not everyone needs all those features of the best AF, highest Fps, virtually unlimited Buffer(with the right card
) and arguably the best ergonomics. Drop any features that are not important and there is a DX model that will fill the gap, at much lower price points. If it is DX you go for.
Decide what you definitely need features wise. Decide FX or DX. Try and handle what you are interested in, which may be difficult atm. Apply a budget, if you have one, for camera
and lenses if you don't already have lenses, and you should be pretty much at the right model for you.
And don't underestimate handling the cameras, as the only reason I started with the D70 ahead of the technically superior Canon 350D, was that it was too small, my knuckled rubbed on the lens, and the D70 felt like it was moulded for my hand.
The Canon was also a bit light and 'plasticy'.
Please let us know what you decide.
PS, if you decide to go new, and for the D500, I was talking about it on another Forum recently and noticed that you can the get the D500 + Nikon 16-80mm VR as a grey import for the same price as the D500 alone in the UK.
Though just had a look and the D500 priced has finally started to come down a bit, and is £90 cheaper for D500 only compared to the D500 + Nikon 16-80mm VR. The D500 + Nikon 16-80mm VR in the UK is £705 more expensive as a combination. The lens on its own is £930
in the UK, so if that is a lens you are interested in, and it is on my camera 80% of the time, then try and get it at the same time as the camera, as savings will be made.
Not to open the 'grey' argument again though, just pointing it out.
So for the long post, but like a lot of people, I am in lock down, and needed a break from the TV.