Replying to Longboarding

Alright here's the deal.  My little brother is buying a longboard.  I know relatively little on the subject, and the rest of our family knows nothing.  So, I come to NSG with questions:

-What makes a good longboard?  Trucks?  Wheels?

-He's about 5'1 and 90 lbs, 13 years old, 14 in December. ..Are there different sizes of longboards?  What size would be good for him?  He can ride a normal skateboard well and do basic tricks, if that makes a difference.

-What effect does board flex have on the ride?

-What are some good brands he should look into?

-Do boards with wheel cutouts make a difference (meaning there's no board over the wheels for deeper turning, I guess)?

-What is a reasonable price for a longboard?

-What other things do you need, like accessories?  Don't say helmet, we have those...

A thank you and +k goes out to anyone who helps out. 
Thanks guys!
:)