Live View is very important for IR work, unless you restrict yourself to 1-2 lenses the focusing behaviour of which you know inside out and thus can adjust to correct focus without further scrutiny. Doable, but certainly slows down the working pace. IR marks on lenses are a reminiscence of the film era and tend to be wildly inaccurate for digital IR in my experience.
I have only used the D300, not the S version, but an additional model letter should not detract from the usability of a nice camera
Bob's last remark reminds me of my D800 which now tends to be reserve to the reserve camera gear so could well be cast into an IR system, if the current PrimaLuce Z5 fails field testing. The D5300 IR has been a useful combination with the D3200 UV as they handle more or less the same and use the same batteries. Add a Df and you are all set to go.
Thus even in the niche sector of IR, there are lots of options to choose from.
