The watts per gallon rule is an antiquated one at best and really doesn't apply any more (I think there's LED systems that power 100g + tanks with as little as 120 watts...). Your best idea is to find out what you want to keep now as it's much easier/cheaper to buy one set of lights at the start that will do whatever it is you want.
At 21" high, your best bet for penetration is going to be with MH or LED I think. Though you could be successful with LPS and SPS with a T5HO as well (SPS would just have to be higher up). A 150 or 250 MH might be your best bet but a 4 x 24W T5HO might also be a good option.
If you plan to do corals, I'd say to buy a decent light right off the bat. Even if you don't add them for a while, it's just a little more money you can try to save.
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