Adam,
There is a big difference in output between a 46 and a 91 DF, and they are very close in price. For a plane that size I'd recommend 91's. After all that work it would be very disappointing if it wouldn't get off the ground. Better safe than sorry, and you can always throttle back.

Check out the Mark Frankel Lear at http://www.modelspecialties.com/Learjet.htm
It's a little bigger than what you have in mind and I believe that his original one flew on two OS77's.

Joe