RE: I know this has been asked many times...
I think you're gonna make her too heavy with a 120 sized engine though... it'll fly like a brick (ok, a brick with alot of power, but still a brick). [&:]
If i were you, I'd build it 1st (with everything you want, but keep it all light as you can at every opportunity), then weigh the plane with everything except an engine in it, then add the weight of a 90SP and the weight of a 120SP into that... then see which you think would be best from a weight standpoint.
It's easy to build a flying brick with enough horsepower... takes a bit more finesse to build an airplane that flies well.
Regards and best wishes.
- Michael