Basically two schools of thought.
Tough, light, and simple: A recurve Excaliber type.
Heavier, faster, little more maintenance and attention: Compound type. Have to watch getting limbs in the string, you don't want the string to come off, bad day for you.
I have an old Barnett Commando that is the recurve type that my daughter shoots. It is slow, but works well. Excaliber makes a scope that you shoot your crossbow through a chronograph and get your speed. Then dial that speed on the scope, sight in the crosshairs at 20 yd, and the triangles are your 30, 40.
All work fine. I would invest in a range finder to get your yardages right on.