LOL, I know the feeling! I started out with a 2.5 gallon tank for my first betta, then saw a second betta I just HAD to have. So I ordered the 5 gal, which will be a divided tank for both boys, with the 2.5 staying around as a hospital/QT tank. But of course when I ordered the 5, I had to find more decorations for it. These two 5-dollar fish are getting pretty pricey. ;)
I kinda guessed at mine based on what's in my 2.5 gallon now. I have one 5-lb. bag in the 2.5 gallon, which gives me about a 3-4 inch layer. So I got a second 5-lb. bag to add to that when the 5 gallon tank comes. If you like a thinner layer of gravel, though, you might stretch a 5-lb. bag -- would probably get about a two-inch layer.
Well sound like you are going down the right path with preplanning. As for the gravel unless you are going to get live plants you shouldn't need that much gravel. And i would recommend putting the conditioner in the bucket then putting the water in the tank.
Welcome to the forum! Looks like you're getting yourself set up nicely!
Don't forget a liquid test kit (they're very imporant), and decor to give your little guy places to hide behind.
Another suggestion, one thats actually helped me a lot, (it's not needed but it's nice to have) is a timer for your light :) (they're pretty cheap at your local hardware store if you can nab one) establishing a consistant night and day cycle is important too~ Especially if you have limited daylight during the winter season!