You don't NEED actinic at all, it's just their to look pretty. How many 'deep water ' corals do you honestly think you see.
With 72 inches I would be looking at 2 or 3 MH. How cheap that will run depends entirely on your d.i.y. skills, and abilities to get used stuff. Lets just say that by being patient my pair of 150's was about 250 dollars.
There are a variety of thing you can keep under regular fluorescents, BUT deep water (>12 inches) will be a problem, you'll need to run carbon 100% and work on keeping your water really clear (easier said than done) and change the bulbs pretty frequently. I keep all sorts of stuff, inc. sps., under 4 * T8 fluorescents over shallowish water. Mushrooms will do very well under this, some LPS will, yellow stars are ok, everything else is a bit of a struggle. Everyone says softies, xenia et al will do ok, well i.m.o. they will survive, but do far better under intense lighting. ..
The main p.i.t.a. with fluorescent (be it NO or PC) against MH is that you spend all your time changing bulbs, and fiddlefaddling with wiring. The upfront cost of MH is pretty hefty, but changing all your fluor bulbs every 4 to 6 months soon gets old, and all the d.i.y. wiring is a pain. Contrast that to 2 or 3 MH bulbs (from ebay !) and far fewer, simpler wiring. The only real alternative if you don't like wiring is one of the super fancy 4 or 6 bulb T5 setups (definitely not the budget numbers, you need one with seagull reflectors), but these are just as pricey as MH! Bear in mind with the T5 luminaires that most of those I've sen for sale in the US are compared to Europe, junk
