The make up of the Scully 280 isn't hard to follow, although there's some parameters and features that don't really apply to it once it's been taken out of it's original tape machine home. It does have both playback and record levelers, but depending on the mode you're in only one will be active at a time. We just use the record leveler for getting a level coming into Pro Tools. I don't think you'd be able to find a manual for this preamp specifically, but perhaps you could for the Scully 280 tape machine.
The sound of the Scully 280 is definitely it's own thing. It's got a vintage, throw back kind of sound and is really cool for warming up a stale sound or for injecting a little color into your tone. It's definitely a specific sort of sound and I don't use them for recording too many things traditionally. Rather I'll run a track back into them to warm them up a bit and/or to add a little life to them. They're cool as traditionally preamps as well, but really only when used in the right situation.
Unless you're working at a professional studio or are a seeker of vintage gear, you're most likely not going to find the Scully 280 around, especially outside of it's original home in a tape machine. These are nice colors to have, but by no stretch are they necessary. In a professional studio they're nice to have around as you never know when you might need a certain color, but they're probably not worth seeking out yourself.