Saga is a great bike for an all rounder. There is no downside to using the group from the donor bike other than the time it takes to remove the parts and install. Just do it, ride it, and replace parts when it breaks. You can also shop around for "better" parts when they go on sale or as funds become available. If you are going to change the wheels (and they don't sound all that exciting from your description), I'd lean towards the Mavic A319 rims as well and probably go with phil hubs. Wheels are the most important investment you'll make (after you've decided on the frame).