If I'm buying windows off the shelf, like Home Depot or Lowes, I go to the store and measure the opening. They will have the exact measurements on the packaging too, but I want to put a tape on them first. Windows are either exact measurements, or they are half an inch less then what they say they are.
A 36 inch wide window could be exactly 36 inches wide, or it could be 35 1/2 inches wide. Whatever the exact width is, I frame the opening half in inch bigger. This gives me a quarter inch on each side to fit the window into the opening, and still have plenty of flange for sealing the window.
I personally like to caulk the seal, and then tape it with ZIP Tape. I also like to use Hardie Screws to attach the window. But that's just me, I really like Hardie Screws!!!
If you are having windows custom made, then just tell them what our openings are and they will order the windows half an inch smaller then your opening. Cost for custom made windows isn't much different then off the shelf.
I personally prefer metal window frames over plastic ones. The argument for plastic, or vinyl, is that it doesn't transfer outside temperatures like metal does. The argument against plastic is that they are ugly, bulky and they don't open as easily as metal windows do.
Be careful of the "extras" that some of the windows try to add on to sell for more money. They will be some kind of energy saving gimmick that you will never notice or see any benefit from. Buy windows based on what you like the look of, the way the window opens, and how many panes of glass you want. 2 pane is the standard, 3 pane will give you a higher R value and sound quality. I've only seen 4 pane once, and that was close to a major airport where the jet engines where very loud.