72G glass tank, weir box for overflow?

I went to a PetCo near my work ... after work. They have a 72 gallon bow front for 199.99 but it doesn't have a built in overflow. The stand is 280. I attempted to find a decent price online but couldn't for the life of me.

I was wondering if it was better to have a built in overflow or weir box.

If I go for a weir box, what is the needed flow rate? What is the going fomula for the flow? Is more always better here? Also how does one prevent an overflow with a weirbox in case of a power outage?

To prevent syphion break, I was going to tie it into a powerhead so the overflow is it is always primed.

For a tank that size, how big should the sump/refugium be? I was thinking like a 10 or 20 gallon, what ever small, cheap tank they have that will fit a euroreef / aquaC skimmer. I know more the better but less cash the better on my checking account. Still in research phase before I buy a single thing.