You're basically buying a hatchling rack already because it only fits 6 quart racks so you shouldn't buy the 12 slot one also, look into buying a rack with tubs for juvenile snakes so when they outgrow this one you can move them. Unless I'm mistaken and other tubs can fit in those shelves it'll only be good for snakes up to around 250 grams. As for your budget you'll probably have to stick to single genes for now. I suggest buying all females and save for a couple months and buy a male further down the line. For $600 you could get a few females. Any of the base morphs like pastel, fire, cinnamon, mojave, spider, and pinstripes can be bought for around $100 each at an expo.
My suggestion for a thermostat is herpstat. Either a herpstat 1 for just the rack or spend the extra money on a 4 for the future.