I wouldn't go with a ton of sand. You really only need maybe 0.5" to 1" of sand if you don't plan on keeping burrowing fishes; anything deeper just makes cleaning a pain even if you have invertebrates that stir the sand a bit. Live sand really isn't necessary, so save some money and use plain aragonite sand; the sand will become live during the cycling process. The bulk of your beneficial bacteria, e.g. biological filtration, is in your live rock, so go for some nice porous dry rock and seed it with a little bit of live rock; websites such as BRS and Reef Cleaners have some really nice dry rock at great prices. With nice porous dry rock, you only need maybe one pound per gallon whereas you'll need much more with the denser live rock that most LFS's and chain stores sell plus you'll be paying for just the rock rather than the inevitable water weight with already live rock. Sumps are where it's at for filtration, help keep the system more stable with the added water volume, and allow you to hide most of your equipment from view; canister filters should be avoided unless you're very diligent about cleaning them thoroughly and often.
If you're looking for low maintenance, then you want to keep your stock pretty low; recommended stocking levels for saltwater are far below those of freshwater. I would keep the 150 gallon if you want to have a tang; they need pretty large aquariums (typically 100+ gallons) because of how active they are. Tangs also are known to be aggressive towards other tangs, so it's best to limit yourself to one per aquarium. I've never found clowns to be hardy in my experience considering that I've been through five of them; of course then, they were all purchased from PetCo, so that could have been part of the problem. Triggers are pretty cool although aggression and nipping can be an issue at times with some species, but they work out well in most cases. Damsels are nice for colorful starter fishes, but they can also be aggressive; they're pretty hardy, though, so they work great for test fishes. I personally favor eels, and you could easily do a smaller eel or two in the aquariums that you're looking at; my snowflake eel is always poking his head out of the rocks plus he's fun to feed and doesn't bother any of the other fishes and invertebrates. Also, be sure to order a bunch of dwarf blue leg hermit crabs and snails for your clean-up crew; they really go a long way for keeping your aquarium nice and clean.
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