First off, like Jeremy said, a treadle hammer is NOT a power hammer. It will take some work off your arms but it's by no means a power hammer. I'm not sure how long you guys have been forging, but you will learn to swing a hammer better, faster, harder etc. After a not to long amount of time, you will be able to move faster and more efficiently on most steel sizes by hand hammer and anvil than on a treadle hammer.
If we knew what design you thought you wanted to work off we would also be able to help you much better. As some treadle hammers can converted over to air in less then 10 minutes, then back a treadle hammer in the amount of time. buying plans would be great in my opinion, many guys that sell you plans would be willing to help you if you are having trouble with something, and may go back to fix something later on and need those same plans. Buying the plans to have the material cut lengths would save you the money many time over if you miss cut a large expensive piece of steel.
Let us know how you get on!