I'll try to answer these questions as best I can- I may be completely wrong though! I researched axolotls when we considered getting one, but decided against it. I'll give it a shot though!
What is the best sized tank for 1 axolotl? I think about a 20 gallon long is a good size for one. I believe a 10 gal is the bare minimum, but read places that it was pretty cramped for big axolotls.
Should they be kept alone or in pairs/groups? They're fine on their own, but can also do well in a pair. However, you'd need a bigger tank for 2 
What do you feed them? I'm pretty sure frozen bloodworms are a good staple in their diet.
What filter do you use/recommend No idea xD I think just a regular flowing filter as opposed to a sponge filter.
Do they need heaters? They actually require really cold water, which is one reason I didn't get one! They need chillers to keep their water cold.
Are they expensive to care for? I think the startup costs are a little expensive, but monthly costs aren't terribly expensive.
Are the hard to care for? Yes and no. They poo a lot, and their water needs to be cleaned daily, but it's not difficult work.
Do you know of any good websites about them? There's a couple good youtube channels if you search "axolotl care", but I don't know of any off the top of my head.