Make sure whatever bag you get has a full zipper so you can use it as a quilt most of the time. That's much more comfortable for someone that tosses and turns.

Sounds like you're going to a wet location, so you may lean toward synthetic fill rather than down since you're a newbie. If you know that you can keep it dry (both in your pack with a compactor trash bag and in your shelter) then I'd go for down.

FYI, Walmart now sells a duck down bag that's likely good to 40F for $70 online. Weighs < 2 lbs.