Definitely got for an NVME/M.2 SSD. My load times in 7 Days are mere seconds versus a few minutes on an HDD. And it helps with performance.
Resolution and size are 100% up to your own eyes. Personally, I don't like 2k and up as a long-term play resolution because too much pretty and sharpness kills my eyes after a bit, I like a happy 720 - 1080 (although 2k/4k/8k is really really pretty lol). As far as refresh rates, FPS, etc., I do recommend at least staying at 60FPS and using something like Freesync or something outside of games if possible. To explain a bit more... maybe this will help ya (yes, it's based on CSGO but the theory is the same, and no you don't need to max your FPS when playing solo stuff or a game like 7 Days, it's just the theory of it all; I normally don't go for videos on research but a lot of people are visual learners and this is a visual thing lol):