Okay, I have been doing rescue for 18 years - both dogs and cats. So here are my thoughts which are biased in some ways. Take which ones you feel are right for you and your situation.
1 - It is always a better idea to have at least two. Then they will always have companionship. They are very social animals and need the interaction. And really two is no more hassle than one. Just a small amount more of food and a little more - er - output but one litter box is enough. Besides, there is very few things that are more fun than watching them play together.
2 - Either sex is great. The problem is if you want to add another kitten/cat later, it may be harder to win over a male to a new addition. Girls do not seem to care, unless they are a Queen B. then they don't want to add anyone ever. My situtation is four cats right now - one male and three females. We call it Booger's harem. Booger has no problem with male fosters being in the house, but they are always rehomed before maturity/hormones hit. Be sure to have them spayed/neutered as soon as possible, expecially if they are litter mates. There is too big of a chance of birth defects if they do mate. Most vets will do pediatric spays/neuters - they younger they are, the easier it is for them. Especially the boys. Most rescues require that they be fixed before leaving for their new homes.
3 - Never declaw. It is a partial amputation of their toes and can effect things like their walking and litter box habits. If you are getting kittens, make it a routine that they get the tip of their claws clipped off every week. You can use pet nail scissors or sharp regular nail clippers. I hold them in my lap and clip all of their front claws. Then they get lots of loving, petting and extra special treats. And this is the only time that they get these special treats - I use freeze-dried shrimp. Everyone has been going through this routine since they arrived in this house and know that it is part of the routine. All of mine have been in my home since they were very young - Booger from 4 weeks, Sissy from 4 days, Maggie from 10 weeks and Bella from 9 weeks. They don't know any other way.
The other option is the Soft Claws but they are temporary and can get very costly. And I think that it is harder to put them on than it is to just clip their nails.
4 - Shorthair -vs- longhair. The choice is yours. I have three shorthairs and the new baby is a Ragdoll, which is a double-coated long hair. Previously, I had a Maine Coon who was also double-coated. Everyone in my house gets brushed during their weekend spay treatment - as my husband calls it. They get their nails trimmed, brushed thoroughly, faces/eyes washed, and ears cleaned/checked. They also get occasional baths, which they are also accustomed too.
~ You didn't ask but thought I would add ~
Make sure that you use a high quality dry food. I personally use Purina One Indoor Formula and Purine One Kitten. The higher the quality, the less residue there is. My guys are spoiled - they always have their main dry food and then they also have 'junk' food - Meow Mix or Whiskars dry. Then they share a can of food every night between the four of them. Sissy is the only one interested in bedtime treats, although Bella is catching on to bedtime treats now. The basics being as long as their main nutrition needs are covered with a high quality dry food, then the goodies are okay.
If I think of anything else, or you have any other questions, feel free to contact me. I have had cats all of my life and I hand-raise orphans (puppies and kittens) for my local shelter. Since we moved to North Carolina 4 years ago, I have raised over 125 kittens from birth to forever homes.