I would take that picture to my local building supply store. There are so many types of windows, they will be able to help you the best.
Also tell them what type of siding is on the outside . Some vinyl windows come with a j-channel already built in.
You should have 14 inches between studs, as long as it is 16 on center.
But be careful, some studs have bows and twists in them. I would remove the sheetrock where they are going to get the right stuff. There could be wiring or even blocking in the wall.