The helmet should be fine but the armor does not look compatible. Most armor that is compatible will have a removable section at the top of the backplate and at the top of the chest plate. I know Troy lee and sixsixone both have models that work with neck braces.
The FOX suit it does not fit without custom modification. No idea about the helmet but most helmets work fine / well enough with the Leatt. Your best bet is to physically try the combo on and see if it fits.
The Leatt Adventure suit fits very well with the brace. I picked up a very lightly used brace for cheap and got the suit on sale, had the small water pack already. I must say the combo fits together very very well. I haven't had the chance to actually ride with it yet but the try-on feel is better than the fox suit that I was riding previously. The only thing I noticed right away that I was prepared for is the lack of head movement, but the restricted movements are directions you shouldn't be moving anyways, so....
Practically all MTB full face helmets will work with the neck brace. I have used an Azonic Fury and now use a Giro Remedy. My buddies are using TLD and 661. The Fox Rampage might even give you a greater degree of movement (especially when you're looking up) since the chin line appears to be higher than other helmets I've used/tried.
FWIW, I also use a Fox roost guard/chest protector that I modified (cut a section in the rear to accomodate for the spine bracket).