1) What to look for (genetic, health, etc):
Well for starter, a good vet is the one that will give your new bunny a check up. He will listen to your new bun's intestinal noise, check teeth, check nails, verify gender etc.
2) Differences between males and females.
I think its a personnal choice. Females are more expensive to spay than males. My male is really nice and affectionnate.... It is hard to tell the gender of a bunny when he/she is too young.
3) Are there many differences between breeds?
Once again it's a personnal choice. If you prefer a small breed, you should go more for a Netherland Dwarf, Hotot Dwarf. Holland Lop, they go up to 4 lbs. Bigger onces as Angora Giant and Chinchila Giant goes over 12 lbs. All rabbit breed makes good pets if they are well taken care of!
4) Anything else you can add
. Yes, read A LOT about a bunnies diet! Read about their needs before getting one. Here are good links: http://www.carrotcafe.com/
5) Questions to ask the seller/breeder.
Pet shops don't know much about bunnies. VERY often they say it's a Dwarf bunnie to get people buying them and OFTEN they grow to be big. So the best thing to do is read about the caracteristics of the breed you want, and look at a lot of pics. Petshops wont fool you if you know what your breed is suppose to look like. Second, DONT take adive from breeders!!! They raise bunnies for production so their diets are different from what a pet bunny should be. They will tell you not to spay/neuter....and we all know that it's the best thing for a pet bunny. They will tell you that using pine or cedar shavingwoods are ok when it's NOT. There again...do a lot of research.
6) Where is the best place to get a bunny?
Probably at a breeders you will get the breed you want. At shelters, you'll have a older bunny that is already spayed/neutered but at least you'll know if he/she is affectionnate. Those bunnies needs lots of love. Then again follow your heart, when you find the right bunny you'll know it. That's how I got mines. I wasnt looking for one and my Bun just took my heart.
Hope this helps.