Welcome aboard.
I believe Sunbird was actually born as a company in 1985 (or at least started producing boats then), and I think I see a hint of that in one of the numbers from your photos.
Sunbirds are mostly just named by their length (175, 190, etc.) and you can find some about their first model year
HERE.
Some suggestions: before y'all get too deep into anything else - see if the hull is solid. Not the fiberglass, but rather (as was mentioned above) the stringers, deck and transom. Those are the "bones" of the boat, and if they're bad you could wind up with danger as opposed to fun.
Once you've confirmed the boat is solid, or you know what's involved in fixing it, then start working on the drive and the trailer - while you're taking a boater safety course. (<== very important!)
There won't be a manual for the boat (per se'). They didn't do that type of thing for boats. You can find a shop manual for your Volvo once you confirm the model number and that will help you repair and maintain the engine/drive. Most of the "rigging" for the boat (seats, steering, bimini, etc ...) are all things that you can replace aftermarket and are not boat specific (even though the manufacturer installed them at production.