Betta filter?

For filters a lot of people have success with the Aquaclear 20 even though it's rated so high. It has an adjustable rate flow and the media is customizable plus the media area is rather large. I would remove the carbon in the filter though, especially if you have plants. If it is a little strong even adjusting it to low, you can easily baffle the output. I like sponge filters with an adjustable rate flow valve for my tanks under 10.

As for salts, unless you are treating for something specific and have no other choice, none are needed nor recommended. Betta do not like the addition of salts into their water, particularly when it raises hardness. I would say never use salt unless you are treating for external parasites and have no other medicine or are using epsom for emergency your-fish-is-going-to-explode, laxative or for removing fluids like with dropsy.

As for diet, New Life Spectrum pellets make a great staple, they are one of the best choices out there. I also like to feed Aqueon betta pellets once or twice a week for variety though it's not as high quality but I like how they keep an all natural ingredient list. You want variety in their diet in order to have proper nutrition. You want to pick your food by how close the protein value is to 45% (NLS has 38% which is good) and you want the first few ingredients listed to have as few plant/veggie matter as possible.

In relative order from good to subpar meaty

ingredients:

Whole fish (especially if it specifies the breed of fish)

Whole shrimps/krill

Fish meal

Shrimp meal

Spirulina (this is an exception to the plant rule since this is a very nutritional seaweed [technically it's a bacteria but they call it a seaweed])

Bad plant ingredients from bad to worse:

Soy based

Anything whole wheat/potato/corn based

Anything regular wheat/potato/corn based

and every food has preservatives so checimals are inevitable. I would advise not to use omega one foods, they have sketchy ingredients listing (and they refuse to clarify how much of the dry weight their ingredients actually weigh) and have a very common occurrence of bad batches in their foods.

I would supplement this with some frozen foods like brine or mysis shrimp or bloodworms, freeze dried works too. I like the Hikari brand of freeze dried foods.

Edit for typo

It is important that you get a filter with an adjustable outflow, so there isn't too much current. The Hagen Marina Slim is a nice one, as is the Aquaclear line.

25 watts would work for a 5 gallon, quality ones allow you to choose the temperature rather than being preset, are shatterproof, and have automatic shutoff. Given these features, there are a lot of nice brands.

Bettas are freshwater fish, and long term use of salt in the water is not good for them. So don't put any salt in the water unless you are needing to treat for an illness for which salt is recommended, and then it shouldn't be used for more than 10 days. As long as she's healthy, she shouldn't have salt in her water.

Bettas are insectivores/carnivores, whereas most fish food is made for omnivores. Because of this, it is best to get food meant specifically for Bettas, since they have more protein in them. Variety is also a good idea. My favorite Betta foods are New Life Spectrum Betta Formula, Omega One Betta Buffet Flakes, and Wardley Betta Food. Giving treats of frozen Brine Shrimp or Bloodworms 2-3 times a week will maximize a Betta's color.