January 25, 2021
What do you need? A used camera with high ISO performance, or a new expensive mirrorless?
Nikon D700 will deliver high quality images up to ISO 6400 if certain conditions exists. Dark areas will show grain and loss of details at ISO 6400. These areas must be lightened to remove grain or darkened to remove details. At high ISO levels I prefer to use bracketing with 0EV, + 1 EV and + 2 EV at 5 FPS.
I am using Adobe Camera Raw/Lightroom Classic for raw file development. Selecting one of the three files that contains details in highlighted areas. See ISO 6400 sample gallery here.
Image after edit in Adobe Lightrooom.
Image as shot with + 2 EV, before edit.