[wonderloss]

A larger tank means higher startup cost, and higher maintenance cost. You need more light, more water movement (either larger or more pumps/powerheads), and more water for water changes.

However, larger tanks mean more options. Larger tanks allow you to keep more or larger fish. There is more room for corals. Also, a deeper tank means greater variety in light level from bottom to top.

You should definitely get a built-in overflow. I also second the advice to plan your equipment with your longterm stocking plan in mind.