Haha, welcome to the forum!
And yes, be careful! Bettas are like Cheese puffs, you can't walk away with just one
Parents...now they can be tough. Mine were more willing because I paid for most of it myself.....but what also helped is the knowledge I gained with research through google, this forum, and a couple other forums. Impress them by researching bettas in-depth and then present what you've learned. That might show them that you have some initiative and really want to take care of your new betta.....just a thought
Personally, I like gravel over sand.... sand is too messy for me, and I'm far too lazy to mess with it to make it look pretty
Though, if your going for an un-filtered tank, I think marbles would be better as you'll be doing frequent 100% water changes and marbles don't go down the drain as easily
However, if your looking to cycle and do weekly partial water changes, go with gravel
Testing also depends on if you want to cycle or not. If you plan to cycle this tank, then you will absolutely without a doubt need a liquid dropper test kit(API brand is good)and its recommended that you test before and after each PWC(after the tank is cycled with your fish of course)...However, I don't do it quite as frequently...but my cycled tanks are pretty well established anyway ^^;
If your not going to cycle, its not TOTALLY necessarily to get a kit...but I'd still highly recommend it. That way if anything DOES happen to your buddy, you can test the water and see if anything is out of whack.
Hope this helps some