You should be able to reuse the case you have as long as it will hold the size board you buy. Streaming 4k locally is fine as long as your hardware supports it but the recommendation is to put 4k in a separate library that is not shared.
You can use whatever case you want. If you use a rack mount case it can also just be set wherever you want it. Even on its side like a tower.
I would say most people would recommend going with the NAS killer 4.0 or 4.1 for media storage and then doing a seperate HP290 for quicksync for the plex server.
Unraid is really popular around here for the NAS. It does cost money but is really worth it in terms of ease of use.
Follow the guide for the NAS Killer, move your media to it. Setup the quicksync box and point it at your media. Lot’s of help around here or the Discord if you need it.