Jeff,
If it actually comes out at 8 lbs. the 91Fx would be OK. However, having helped someone with one of these planes I don't know how easy that would be. We ended up using a 120Ax and it came out around 9lbs. There's no such thing as too much power in a pattern plane. You don't use the excessive power to fly fast, but rather to improve the response. With a 120Ax you will almost never use full throttle for any extended period of time. If it's windy out, you may need to apply full throttle briefly however. If you're just starting out you will probably be flying Sportsman. The Sportsman sequence is not as demanding on the airplane and power system as are the 402-404 schedules. Having flown my friend's Venus 2 with the 120 and also a Groovy 90 with an OS 91Fx I think you'll be OK with the 91 for Sportsman IF YOU KEEP THE WEIGHT DOWN. The problem you'll run into with the Venus is that they seem to turn out nose heavy. The 91Fx will help here. Plan your servo installation accordingly. You may find that you need to put all the servos in the tail as suggested in the instructions. I would say go with the Venus 2 and the OS91Fx. The 120 is almost a bolt-in so if you find that you need more power, you can always get one later. In the meantime you can fly with what you've got. The Venus flies great - very little mixing required to make it almost perfectly neutral. If you like to set things up nose heavy, you may need some rudder-elevator mixing for knife edge, but it has almost no roll coupling tendencies. A very good, honest airplane.

John Pavlick
NSRCA 3815