7 year old wants fish!

So my son has been asking for fish ever since his Betta fish died, it did live for over 3 years. I don’t know anything about fish other than Nemo has to go in a saltwater tank. I’ve looked in fishlore and scrolled through some topics here but I’m still as lost as before any reading. We went to a pet store today and they showed us a 21 gallon kit with a bow front that included:

• FULLY ADJUSTABLE LED FIXTURE WITH WHITE & 3-COLOR RGB LEDS • INFRARED REMOTE CONTROL FOR ADJUSTING LED COLOR & INTENSITY • 171.7-GPH WATER PUMP • FILTER CARTRIDGE WITH PREMIUM CARBON

I don’t know what any of that means but considering the price compared to a bare tank, I figured a kit for how expensive it was, is the way to go for a novice because of the convenience. I have since read that most kits have filters that need upgrades, so I’d need to change that. Not sure how true that is?

My question is, should I go with a kit? or put something together myself? If so, please help. I was expecting to pay several hundred on a 20-30 gallon tank with all that’s needed for it, and several hundred more for the six types of fish he wants, although he doesn’t know what six, not sure where he got that number from. lol

I wanted to just setup the tank for him and in a few days/weeks take him to get an array of fish. Does that make sense? What can I get with about $1000 including a stand, tank and fish? TIA.