Mid-range windows with dual glazed low-e and argon gas should usually have u-values between .30 and .35 Anything with higher values I would not choose. (The lower the U-value the better)
There are many things to consider along with u-values. Air infiltration ratings, the type of spacer, how many layers of weatherstripping and several other factors all are in the mix to look at and compare. I would also pay close attention to the window choices of the pros here, who after all have installed thousands various brand windows and know the windows inside and out.
On most aluminum windows, nothing else comes out except for the old aluminum window. Sometimes the old frames are even left in depending on the siding material and flashing system on the exterior of the home. Stucco would be one such case.
Capping and wrapping are the same thing. This is custom bent painted aluminum that is formed to fit on the exterior of your old window sills and casings. On aluminum windows, you may or may not need this depending on how your old windows were finished off on the outside.
I am posting before and after photos of windows I installed and wrapped for Warren Buffet's son Howard. On the first photo, they were installed but not wrapped yet.
The second photo is after wrapping them. This should give you an idea of what we do when we wrap or cap a window.