I used to own a cellular model, but then I realized that 95% of the time I use it at home, sometimes in the office and if I’m traveling, usually the hotel/coffee shop has WiFi, and for the 5% remaining, a hotspot from my iPhone is good enough. For my use cases, a cellular model is not worth the price difference, although it is extremely convenient for sure.
Regarding storage, if you plan to use it just for PDFs and don’t intend to keep large video files on it (you can stream pretty much anything these days) then I guess a base model of 256 GB will be good enough for you (it is for me), everything else can just be uploaded to iCloud and downloaded over WiFi when needed.
As for the screen protect, I’ve never used it so I can’t comment on that part. I’ve never seen any screen scratching after intensive Apple Pencil usage, and I’m not going to degrade the gorgeous mini-LED 12.9” screen I paid good money for by putting a screen protector all over it.