Marina LED Aquarium Kit, 5 Gallon

Q: Anything else you need to buy with this kit to get started? Thermometer, testing strips, anything like that? Or is it really all-inclusive?

You don't have to buy those things to set it up and get started, it comes ready out of the box. however once you set it up those items are worth purchasing. After I set mine up I got a stick on thermometer wish is pretty cheap and easy to install and also a heater to control the water temp. You may or may not need a heater, depends on your average room temp and the temp specs of species of fish you have in the tank. For instances, if your fish need to be in water with a temp of around 75° and the room temp where the aquarium is located is on average around 75° you shouldn't need a heater. Hope you enjoy your new tank.

Q: is there anything i need to watch for with my betta ?

This is a perfect tank for a betta IMO. Only suggestion is to keep the filter outflow on the lowest setting or your betta will be buffeted around. I spent about a hundred hours researching tanks and didn't want to get this one because the filter hangs off the middle of the back, obscuring one whole side view (my tank is on a shelf that is open to two rooms, so no "back' for the tank). I made some adjustments and tossed the original lid and was able to hang the filter off of the more narrow "side." It's perfect for my betta Simon. He loves it, I love it!!