Add about 4 lbs for a fan, motor and battery, for an AUW of 12 lbs.

A good rule of thumb is 100 watts/lb for good performance so your looking at 1200 watts.

This will be a 90mm fan. Using a 5S lipo, the current draw will be about 65 amps, a bit much for my taste.

Going to 6S, the current drops to 54 amps (1200/(6*3.7)). With a 6Sx5000 mah pack, nominal flight time is 5.5 minutes (5*60/54).

For vertical performance you would need a 1:1 thrust ratio (12 lbs), which might be hard to do with a 90mm fan, but not impossible. If you don't need vertical then a thrust/weight of 0.6 (~7 lbs) would be fine for decent performance.

I suggest you look at Electric Jet Factory (www.ejf.com) and have a conversation with Robert Wagoner. cmreefer's suggestion is right on the money as well.