Hey.... congrats on aquiring a fish only slightly less nosey than a mother in law!! JK... well... kinda.
You can keep a betta in a container less than 1 gallon. The biggest thing with bettas and any fish for that matter is water quality. You can keep it in a 10g... but if the water sucks it is no better than a cup.
That paticular set up is fine for a display.... but really it is a bit cramped and made for looks and to be looked at rather than what is best for the betta. They sure did write a convincing sales pitch though.
If the room/ area you are keeping him in is warm all the time... you can skip a heater. To check... floating thermometer is usually around $2. In the 70's is good.
Water conditioners are mandatory. If you are staying with that particular set up.... daily water changes will be needed. There are several different ones on the market. All depends on what you want to use. Prime, Amquel & Novaqua are ones I like. Roughly... 2 drops of Prime or A & N per gallon. They are a concentrate to remove or detoxify trace harmful elements in water. Let the water sit overnight to get to room temp. Ideally when you change the water it is to new water of the same temp to minimize stress.
Food variety is good. They seem to develop their own tastes. Same food all the time and constipation can happen.... but don't worry about that just yet. Big thing is when feeding pellets.... they swell after they have been eaten or sitting in the tank. Some bettas eat fast and clamp afterwards..... bellyache. It is like Thanksgiving expanded to double the size after you ate your fill. Best to feed little bit through out the day if possible.
Lots of great sites.... hang in there. They are very addicting and fun fish!