Couple of things to consider.
That's a smaller tank, so a 250w MH will likely introduce heat issues. How will you deal with that?
"High flow" is subjective. With a 15g sump, you're not going to pump a lot of flow using only your return pump. Other options are in-tank pumps or closed loop. With a lot of flow comes substrate concerns...are you wanting sand? Might be a storm
Clean water and proper levels of Ca/alk will give you great growth coupled with good enough lighting (250w would do it). Some people like to dose Mg, so test and see if you need it once your corals start growing.
Keeping these levels comes with options. Ca reactor, dosing pumps, kalk top-off or manual dosing. You can spend 20 bucks or 800, more money isn't better, just sometimes easier.
Evaporation with that size tank using MH will be a couple of gallons a day, how will you top-off to maintain SG. Again, you can go cheap with float valve and jugs, manually top it off everyday (gets old fast) or go with ATO.
That's just a couple of things to consider. Pretty much figure out how much you have to spend, then determine the best way to spend it.