You're going to be doing water changes, buying 'betta water' isn't practical. buy some water conditioner(a lot of people like Prime, I use AmQuel plus as it has an ammonia detoxifier).
your 5 gallon will start to cycle with a filter in there so you should buy a test kit:
link to one.
this way you can monitor the process and do water changes as needed. read the sticky on cycling, under "fish-in" as your betta will be in there.
Unless you plan to run the tank for a few weeks without him.
I would also recommend a plant. Something that will be ok in low-medium light but with broad leaves for him to rest on. Mine enjoys red ludwiggia.
Moss of some mind might also be good, mine enjoy playing in it.