Depend the current area have enough sitting for the aluminum, if not you may need to hack the grill out, or maybe add some brick/concrete to let the windows sit on.

Usually calculation is round up. 3.5 or 3.8 use 4', 4'8" use 5'

Better ask the windows guy come over and check, they can advice better.

I also entire house windows from few windows supplier, they too tell me per sqft price ask me calculate myself. If accept only go measure and discuss detail. That is very standard practice. I don't think is rude.

They only do exact measurement when price confirm. Anyway few i ask round the measurement. If you have 5.5', is 6' so calculation is not hard. You need to provide the opening, if not they maybe extra charges. If your wall is already plaster they also highlight installation may not look nice.

Provide opening = enough space for the windows. Or they may charge you removal of the grills or old windows.

Plaster = After brick work, you need to put cement to make it like normal wall, this is call plastering.

Usually window frame are install after brick work, because they need to mount the windows frame holder / bracket to the brick, then the wet work contractor will do plaster. This look nicer. If you have already plaster they may need to hack and put the bracket and plaster back. So this may not turn out to be nice and may charge you extra.