They can't reach Buddy Belts so they won't be able to chew on it if you left it on. They're pricey but very long lasting ($50+). Buddy Belt came out with a slightly lower cost harness (~ $20), same style, but made with pleather instead of real leather.
You should visit Maximo's website - she sells them!
We all swear by Buddy Belts.
As for leash - I only use leather leashes. If you ever visit a dog show in your area, they always have booths that sell them.