In a puppy, I would want to see the parents run, that way you can be "sure" it's coming from good hunting stock. If you have any buddies that have rabbit dogs, ask what speed their dogs run. Speed is relative, but if you run w/ some buddies who have dogs, you will want your dog running the same speed. Faster isn't always better, I run 2 packs, on a scale of 1-10, my A pack runs speed 8.5 and my B pack runs a speed 7. It's enough of a difference that I don't run them together.
You will also want to check the puppy AND PARENTS for deformities and conformation (good looks). Some things to look for are crooked backs, offset tails, knobby kneed front legs, straight stance, over/under bites, etc. etc. you get the point. I'd recommend getting the pup around 8 weeks old, give or take a week. 8 weeks is a good age, it's given the pup time to socialize some w/ it's mother and siblings, yet it's still real young and will take to you and your family like a kid in a candy shop after a day or so when it realizes it's new home.
Honestly, if you find a dog out of good hunting stock, you will probably be all right. I usually overdo it, as you can see