I'm guessing your either planning to get water dechlorinator/conditioner? If not that is a must. I would also reccomend getting something to cover the bottom of the tank, like rocks or gravel, a net, a small betta bowl (1-2 gallons)for water changes or as an emergency tank (never know what may happen), test stripes to keep an eye on the water levels (though not needed, but are handy when trying to diagnose causes of illness), and live plants are also a good thing to have as they add oxygen to the water, but not needed.
Like was said before i wouldnt use bettafix unless it is a must. Most things like fin rot can be cured fairly simply with a little aquarium salt (i've been astonished by its wonders).
When choosing a good betta, just look for a healthy one, no ragged fins or dark edges, not lethargic, or sickly looking.
Sounds like you're pretty prepared though, if theres anything else i think of thats handy to have i'll post back. :)
peace. love. & bettas.