I haven't had a huge amount of experience with octaves, but I am VERY familiar with normal mandolins. Most of my experiences applies to both, hopefully! First, you are going to have to decide how much you really want one. In other words, how much are you willing to pay.
-In the mandolin family, decent quality instruments start around the 1k mark. Professional quality instruments range for 3-10+k.
-When comes to the two different styles (A vs. F) there is not really much difference in the sound. You will pay considerably more for an F-style, however.
-The first thing to do with ANY mandolin you buy is take it to a qualified, MANDOLIN repair guy and have it set up. If you have a mando that has not been set up, or has been set up poorly it will shred you
fingers.
Only your ears and fingers can tell a good one from a bad one. Watch for obvious things like cracks, buzzing frets, bad setup, poor quality, etc. I can recommend John Gathright of Gathright custom mandolins in Foreman, Arkansas. He makes super good quality instruments for reasonable prices. If you are willing to spend over three-thousand pm me and I'll give you contact info.