1) 5 gallons is fine for a betta.
2) A 50 watt heater is best. I use the Zacro heater in my betta tank, and it works great.
You will need a lid on the tank. Not only will this help with the evaporation, but bettas are also great jumpers.
3) Sponge filters are best, but as long as it doesn't have too much flow, it will be fine.
4) This question was already answered, but I recommend getting some plants with broad leaves for your betta to rest on. I would also recommend the betta hammock. My betta sleeps on his every night.
6) Bettas can get fin rot. But, this is easily treatable with some aquarium salt, as long as you catch it early enough.
As Adorabelle said, bettas love to have things that they can swim into. In my profile pic, you can see my betta in his little temple. Bettas also like tight spaces. You shouldn't put in anything so tight that he could get stuck, but something small enough to make him feel nice and secure.
Here is a pic of my tank. It's an old photo, so it looks a little different now, but it gives you an idea for what I mean with the hides.
(Underneath the temple is a small clay pot that has one flat side. The entrance is partially covered by plants, but my betta loves to sit in there.)
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