wat kind of piping will u be using? size? i suggest that for the overflow, use at least 1.25inches, the bigger the better so that it will be sufficient no matter how strong your return pump is...
for a 3ft tall tank, i believe u might need at least 400W MH to be able to blast thru the water and reach the bottom of the tank...i suggest that u use a shorter tank, at such height it will be very dificult to place corals on the sand bed and it is not cost effective, 400W MH will be very very hot and u will need a strong chiller...