Decide what look you like - black sand? white sand? brownish gravel? there are a lot of choices (and prices!) out there.
I have flourite in both tanks. It's what I started with a few years ago and my plants are fine.
I don't run CO2 'cause I didn't want to go that route. I dose liquid ferts 'cause my tank is only a 20L it's not too expensive.
Used equipment is fine although one of the small kits might be easier.
I'm glad you're looking after your betta