Janet,
I'll toss in my welcome, to both RCU and RC flight. Great bunch of people and we're (more than) willing to give advice.

I'd make one observation; small RC aircraft do not fly very well, at least outdoors. They're generally tossed about badly by wind, so the larger wingspan may be advisable until you've learned.

I think electric power is ideal, especially for converted free flight planes. That Prarie Bird 50 could most likely be pulled by an inexpensive 05 size electric motor. (I'm presuming that you're not interested in spending $160+ for the motor and ESC, plus another $50 to $100 for the battery packs(s).)

If the plane can be kept to 20 to 24 ounces AUW (All Up Weight) then a speed 400 size motor would pull it fine. (With electrics you can usually expect at least 1/2 your weight to be in the flight equipment.)

I'm hoping that some of this info will be helpful, but I'm going to stop for now. (I may be telling you stuff you're fully aware of. )

In any case, welcome.
Dennis-