Welcome to RS, and Congrats.
I'm a beginner myself so can't give you much advise. It will be easier and cheaper to find an used tank and convert it into a sump/refugium than to buy a prefab sump. It all depends on the height of your stand but I would put the biggest tank that would fit underneath, and just add baffles to it.
What kind of fish and coral did you want to keep in your tank?
GOOD LUCK!