$100 is more than enough to get together a tank thats nice, if your alright with square box tank, not a plastic moulded one
shame to hear that the pet shop in papakura closed. animal shack in pukekohe is a great one to go to, havnt been to dury for a while but last i went 2 years ago it was a bit of a dive.
a heater can be cheep or expensive, the cheapest ive found is from "the fish tank"
removing everything etc to clean the tank is bad, it can kill all the good bacteria in your tank and shock your fish, best to change about 30% of the water every week, a siphon is just a length of hose from plastic box, bunnings etc and costs less than $5
bettamin = fish food for fighters, normal flake will feed fighters too.