Welcome to the forum, and congratulations on your new purchase. She looks like a beauty...the late-model 280 with a 5 speed is a desirable combination for a street car that gets driven often. While not as functional as the usual heavy 5mph bumpers, the thin bumpers are much nicer looking IMHO. I'd be interested to know what differential the car has (an R200 would be nice, but certainly not necessary).
Unless you're planning to upgrade the front brakes and get a larger master cylinder (and track the car), rear disc brakes are really not necessary. The stock aluminum drum brakes on the rear are more than adequate for any street driving...and even the occasional auto-x or time trial event. There are several good discussions here on how to maximize performance from the stock rear drums.
Lots of good forums and groups on social media for the S30; but another forum I like is ClassicZCars.com, especially for information on original restorations.
Good luck with your planned mods & upgrades, and please keep the updates coming!! 👍