DIY sump and overflow questions

I'm going to buld a sump/fuge for my tank but I'm not sure how to go about doing it. I think I'm going to go with a 20 gal sump. For the overflow, what would you say would be the best idea, drilling into the tank, an HOB overflow box, or an overflow made from pic pipes. Since this sump is going to have a refugium, should I place all the equipment before it, or after? Some people have an ATO in their sump, is it really necessary? And when topping off, I would do that in the sump right?