I've flown the Bobcat XL, Kingcat, and Comp Bobcat. Comp Bobcat flies like the Kingcat, just lighter and lands the best of all three. The composite Bobcat flies better than the XL, is lighter than the XL, lands better than the XL, bigger than the XL, more room inside, and requires no gyro. It is overpowered with a p120, but going down to a P80 or P70 would make no sense for the weight issue. The only real gain would be to go to a P60, but in your instance I think you are going to need more thrust. BV has a Comp with a tuned in P60 that flies awesome, but thats at sea level. I have no experience with the wren. An engine that weighed the same as a P60 with 18lbs of thrust would be the ultimate for this airplane.
You will love the airplane, it goes together very, very quickly and alot of fun to fly.
I think if you go with the smaller engine, I would leave out the cockpit kit, smooth stop, air gauges, and use duralites.
Scott