Wood trim total replacement

I have a brick house that has wood trim at some points (At the entryway, behind the house at a bathroom window and above the garage). The boards (roughly 1x2s) are rotting; especially along the back side of the house, and I don't think it's worth trying to repair them. The rear window has a wood trim area roughly 6' wide and 7' high (excluding the window itself).
The wood looks like: http://www.thisoldhouse.com/toh/knowhow/_repair/article/0,16417,680938-3,00.html # except that nearly all boards have a certain amount of rot.
I gather that the proper procedure for replacing the wood trim is:
A) Decide on the type of wood I want to use to replace (size, etc). B) Purchase new boards (1x2s and 1x4s, I presume though I will measure them and know 1x2 does not mean 1inch by 2 inches). C) Treat the wood before installation. D) Yank old trim wood. Assuming the wood behind the trim wood isn't damaged, I presume I can reuse it. E) Install the new trim wood, using a level to make sure I'm aligned/etc. F) Paint.
Questions:
A) What sort of boards should I use to replace the existing boards? Is Home Depot the appropriate place to get replacement wood? Obviously I need this wood to be very straight as I don't want it to 'warp'. Is that a major concern? Would I be better off with a lumberyard? B) How should I treat the wood before installing? The ThisOldHouse website suggests Borate but that might be hard to get. Is there a primer one recommends? C) What should be between the 'backer board' and the replacement wood? Do I need any special insulation paper/tar paper? Should I use a polyurethane glue at each of the board joints? D) Are screws or nails adequate for this type of job? It looks like the original wood has some type of staple. I like screws myself but realize they may look a little sloppy for exterior work, though I'll have to paint anyway.
Thanks,
Roger
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Art makes a good point: Before replacing the trim, make sure there is not some condition that promotes rot.
I'm an architect documenting and proposing repairs on a series of residential failures. This is what I suggest: Go to a lumber yard where the folks have experience and are willing to share it. Use treated wood that is kiln dried after treatment or good quality wood that you treat after cutting it. We use Southern Yellow Pine, and there are other good woods. Paint ALL surfaces of the wood. The end grain picks up moisture faster than other surfaces. You can't assume the joints will stay tight. Since you are doing the work, you can afford the time. Use stainless steel nails. There should be some sort of weather barrier. Look when you open a test piece of trim. TB
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The bad news is that incorrect brick installation may be causing your problems. Are there weep holes above each window and door? Do you know whether they were properly flashed?

http://www.thisoldhouse.com/toh/knowhow/_repair/article/0,16417,680938-3,00.html #
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The house is nearly 25 years old, so I'm not surprised some of the wood is starting to rot. Actually it was somewhat rotted when I bought the place and the inspector seemed to indicate it was just normal. I suspect it's just cheap wood. I live just outside of Houston which is prone to high humidity.
There are weep holes running the bottom of the house.
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