While I'm a huge T5 fan, the MH route does give you considerable flexibility in using them over nearly any size tank. You can scale easily by just getting another ballast and pendant.
Remember, the total wattage doesn't really mean much. I've kept BTA's under 4x39W before, but the tank was small (58ga) and I had individual reflectors, good bulbs, and active cooling - and the nems were all up relatively high in the tank. I say this to show it's not as easy as getting more wattage and you're fine.
Do some front-end research (pain in the butt and tedious maybe) and it'll save you a lot of headaches in the future. For example - try to find out the difference between a $250 4x39W T5 fixture, and one that is $600. What features are different, what does each feature do, how relevant is it to your setup.......Once you can tell the difference, you will be better able to make your own decisions.
oh, and +1 to SkullV about the lights/stocking list ideas.