The Vortech's are worth every penny. They are simply awesome and they move a ton of water. If you ever need customer support, they are top notch.
In a 90, depending upon your tank, 1 Mp40 might be enough. Certainly 2 would be great. While the wave action is cool, I run all of mine either on reefcrest or full constant. I'm moving 300 gallons across an 8' tank and I prefer more confusion in the flow pattern.
Having the motor outside of the tank will also keep heat out of the tank, and you would be surprised how much heat these little pumps make. If you havent seen how they run, check out some of the videos on YouTube.
If you have any specific questions, feel free to ask.