If you buy a hatchling you will have to buy a small tank and then get a bigger one as it get's older. You could get a full size tank, so you don't have to buy another, but would have to partition it. A lil baby corn would feel lost in a full size tank!! A general rule I go by is that you shoud get a tank that is long enough to let your snake stretch out fully. (they can grow to about 5')
As for other things you need: you already have the heat mat so you'll need a thermostat to keep temps stable, a thermometer to measure temps, a water dish, hides (bark, toilet rolls, cocohuts, ice cream tubs etc etc), substrate (woodchip stuff or keep it cheap and use paper towels) & mice!
As for red phased corns: there are quite few different 'red phases'. You have your classic normal phase (red n black), amelanistic (red with no black), bloodreds, crimsons, sun glows, candy canes etc etc. Have a look on the photo gallery and you will get an idea of what all the morphs look like.
Good luck!