Gas or Electric?
There is a darn good reason why forty sized glow powered airplanes are the most popular. They are the best compromise between the easy of flying a very large plane and the economy of a small one.
If you are serious about learning to fly, learning to run and maintain an RC engine, and learning all of the skills and techniques to make you a skilled modeler, then the engine to start with is one of the plain bearing .40 glow engine.
My personal preference is for OS because they last for bloody ever, and are very easy to tune and maintain, but the other manufactureres make some good offerings too. I will say this, while there are guys that swear by some of the other makes and complain that the OS engines cost too much. I have never spent more than 2 minutes fiddling with an OS engine.
The best value in beginner engines in my opinion, is the O.S. LA-.40. I bought one last january to run in the club trainer and it has had more than 15 gallons of fuel through it, is still on the original glow plug, and has operated with out a hitch in weather ranging from below zero to over 100* F. I've run it in freezing rain, snow, and fog, and on hot muggy mornings. I have had exactly ONE dead stick landing with it, and that was due to a fuel line problem.
Electric is now a viable option, with the cost slowly declining, but running them is still more complex than flying glow. The cost is competitive with glow only because you do not have the ongoing expense of fuel, but the entry fee is a bit more than what it takes to do glow.
For a properly equipped trainer with a .40 glow engine, be prepared to spend about $400 for the model, radio, engine, and sufficient tools and support equipment to fly. (Kadet LT-25 or LT-40, OS-LA.40, four channel radio, covering and accessories, fuel and AMA membership.
If you go electric add $200 (SR Cutie with SR's recomended motor, spare battery pack, charger, radio and misc. support equipment)