What power source should I use?
You're going to pay for bleeding edge technology, that's for sure.
A 7.4 LiPoly pack is actually a fair equivalent to a 7-cell NiMH pack, when properly sized to handle the current required by the motor. At full charge, they are actually 8.4V, and hold their voltage better under load. However, as you said, they are VERY expensive, and I recommend against newbies using LiPolys, because you need to treat them special. Newbies are bound to ruin a pack or two in the learning process, and I feel it's much better to learn on something that isn't so expensive and is less likely to redecorate your house if you mistreat it.
The 3000mAh NiMH cells are a good choice, but they are a fair bit heavier than CP1700s. I don't think they'll impact the flight characteristics of the plane much, though. They ought to fit in a "flat" configuration (side by side) and should give you fairly impressive flight times. Adding an 8th cell will give you about 25% more top-end power, and would allow you to run at a lower throttle setting, extending your flight times.