Welcome to the forum! I'm glad you're asking questions before going out and buying. I'll answer your questions to the best of my ability. :3
1- Bettas aren't really hard to care for, if that's a good enough answer. A three gallon tank will suit one just fine. All you'll need is a heater. In a tank that size, you may or may not want a filter. But they can live just fine in a smaller tank without one provided the proper water changes are made.
2- I recently went on vacation and left five of my bettas at home for exactly seven days. I only brought the one because he needs special treatment water for a chronic illness. All of them were fine without food and without their water being changed for that amount of time, but granted, they are in ten and twenty gallon aquariums.
3- Bettas are freshwater fish. The only special thing their water needs is a heater. I only mentioned the treatment water because one of my fish has chronic SBD. Most are healthy, though, so I highly doubt you'll run into any problems. :3
4- People feed homemade food, live food such as brine shrimp and various worms, and frozen or freeze dried foods such as bloodworms and shrimp and krill. As long as the food is high in protien, it's good.
5- It depends on who you're asking. My female is the most attentive and sweet out of all my bettas. The males have longer fins and some people think are prettier to watch. As a pet, I don't think you can go wrong, either way.
6- The only way to put two in one tank is to divide the tank. They're very agressive little things. I wouldn't recommend dividing a three gallon aquarium, since it doesn't seem that it would leave too much swimming room. My divided tanks are both successful, after having worked out many a kink along the way. Lol. I would say unless you're buying another tank, only get one betta.
“Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.” - Albert Einstein