I am going for a true mixed reef.
For ballasts are the stock ones on fixtures usually good or do some need upgrading?
Also what is overdriving lights? and would a lower end fixture with higher end bulbs and individual reflectors be the same as just going for a "high-end" setup?
stock ballasts are almost always fine... as long as you don't get a dirt cheap fixture the ballast wont need upgrading.
Overdriving t5's is referring mainly to using t5's on an icecap 660 or 430 ballast.... it "overdrives" the t5's to give off more intense light... and it works well, but there are a few disadvantaged to it..
1. It will cost you more, the ballasts to overdrive are expensive
2. The bulbs and fixture will run hotter, because more wattage is pushed through the bulbs
3. Because the bulbs run hotter, they will burn out quicker and need to be replaced more often
4. The color temperature (K rating) or the bulb will change because it's not being driven to normal wattages.
In my opinion over driving t5's on a 72 gallon bowfront doesn't make much sense... unless you really wanted to do an icecap retrofit with icecap ballasts and have an SPS dominated tank.
For a true mixed reef tank my recommendation stands at the Aquactinic tx5 fixture...
Nova Extreme's are deceptive... you think you are getting a good deal because the bulbs are included in the fixture when you purchase it... but these bulbs are crap... burn out fast, don't give out much light, and look crappy as well... all and all it's not a horrible fixture.. but there's a reason why it's cheaper than the rest..
also, for the 48" versions of the nova extreme, they don't offer a 5,6, or 7 bulb fixture... they only offer a 4 or 8 bulb fixture... 4 is to little, and 8 is too much.
I might not be giving the tek t5 light fixture enough credit.. as it does have good reflectors and ballasts, however you NEED to purchase an acrylic splash shield for it separately.. it is also not actively cooled, so bulb output is decreased over time, and bulbs burn out quicker.
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