If I were you, I'd go with a glass 75 gallon tank, instead. IMO, glass vs. acrylic is a personal choice and a lot of people really like acrylic. Acrylic is stronger, lighter, has a higher insulation value. Some say that acrylic is more clear than glass, but I would question that claim. Acrylic scratches MUCH easier than glass. No matter how careful you are, you WILL scratch an acrylic tank. Once scratched, it really interferes with viewing pleasure. Especially when algae starts growing in the scratches. Acrylic tanks also have a top that I hate!! They offer you two square holes, to access the tank through, usually.
The tank you're looking at has a front to back dimension of 15". You'll grow to hate this!! A 75 gives you 18", which is only 3" more, but will drastically improve your ability to aquascape. A 75 glass will also allow you to go with a 6 bulb Tek5, just fine. The 6 bulb fixture will allow you more flexibility with future upgrades and will grow more corals than a 4 bulb fixture.
Yes, you can suspend the fixture above the tank, but the higher you suspend it, the more light you lose.
As for your question about 10K or 12K...10K is going to be more white and 12k a lil' more blue.
With T5HO lighting, you have way more flexibility in color than you do with any other lighting. For instance, in my 6 bulb Tek5, I have 4 different color bulbs, to create the color in the tank, that I want. Check out the bulb selection at reefgeek and you'll get an idea what I mean.
