You absolutely MUST have a sump. There are no problems associated with flooding unless the tank is poorly designed or constructed.
There is a thread running on this forum on proper sump design. Check that out for ideas. A visit to someones tank will clear up a lot of questions, and would definitely be worth while.
With halides, it's more a matter of keeping things cool, rather than heating.
If you are getting the tank custom build, add a lot of closed loops. This helps to reduce the need for unsightly powerheads scattered throughout the tank.
Some things you can do in stages, some you can't.
Get all the pluming done up front well before filling with saltwater. Test with freshwater and make sure you are happy with it before filling with salt water. Get all lighting sorted before filling with water.
Get all live rock aquascaped the way you like it before adding livestock. It's much harder to change this with a tank full of coral.
Allow the tank to cycle for months with live / dead rock, and add livestock slowly initially. It takes a lot longer for a marine tank to reach equilibrium, and it is far more important than with freshwater that it does.
Equipment which can be added later for convenience include:
Calcium Reactor
Denitrifier
Aquarium Controller