Well, I don't have a bunny, but I can answer some of your questions.
Yes it's okay to keep a bunny indoors all the time. Actually, I think it's safer that way: the bunny isn't exposed to extreme weather, insects, predators, stupid neighbours etc. Plus you get to see him/her more often and snuggle them more
You'll just need to bunny proof your house, i.e. secure electrical cords, covering up outlets, etc.
As far as being sanitary, as long as you clean out the cage once a week, there shouldn't be a problem.
Yes bunnies can be potty trained. The rabbits I took care of at the shelter used a litter box all the time for peeing in. Not so much for the poops, but those are easy to clean up.
A shelter or rabbit rescue would be the best place to get a bunny. Don't trust pet stores! Not only do they contribute to pet overpopulation by supporting breeders, pet store animals are usually not well looked after. They can have a variety of health problems, and depending on how stupid the staff is, could even come to you pregnant. So I definately recommend either a shelter or a rescue.
Lots of times shelters will have rabbits, but they don't advertise them like they do their dogs and cats. So call around.
As far as general husbandry, I would recommend that the largest cage you can afford. I hate to see bunnies stuck in tiny cages. Also, do not get a cage with a wire floor, as these can lead to foot problems. They will also need floor time and cuddle time
Never use cedar or non-kiln dried for their litter, as studies have shown these can lead to health problems. I would recommend Care Fresh.
For food, good quality rabbit pellets, fresh veggies, and HAY! Hay is really important for roughage, and for their teeth.
They'll need a little house that they can hide in, a wood block to chew on, and toys.
Also, you may want to look for a rabbit that has been neutered or spayed. They tend to be less aggressive, more friendly, easier to litter train, and live longer. And unless your rabbit is altered, getting a friend would be difficult; same sex because they might fight a lot, different sex because you'd have loads of babies on your hands. If you can't find one that's already been altered, the surgery is relatively as safe and easy as it is for dogs and cats.
I love bunnies, and was very tempted to adopt one we had at our shelter, but with 2 guineas, a cat and a dog, our apartment is kind of full. They are wonderful little guys, fun to cuddle with, and fun to watch hopping around.
Good luck with your decision. I know Avalon has a bunny, maybe she'll post about her experiences.