If you can post a picture, it may help us to get a better idea. From the sounds of it, you will be painting this, correct? If so, then you could really use any wood here. Anything from pine, douglas fir, etc. all the way up to hardwoods could be used. It really is up to you. Since you will be painting it, I wouldn't put an expensive hardwood there though. Just make sure you design it in such a way that rain runs off and seal every possible connection point, otherwise water will get in and rot the wood. Without a picture, I can't know the design of this window for sure, but if possible you might want to consider adding some kind of air gap or insulation between the inside and outside. You are replacing the windows with dual plane which will help, but if you have solid wood runners going from inside to outside encasing the windows, you will get thermal conduction through the wood. A simple air gap between inside and outside wood members could give a welcome extra bit of insulation to the overall window opening. Just a thought...
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