If you are installing windows, frames and all, not just window sash replacements, I would want to get references from previous customers because those kinds of installations can be complicated and tricky. You could wind up with bad leaks and other headaches. If you are concerned that a wall into which the window is to be installed is problematic, then you definitely need a general contractor (one who comes recommended, if you can find such a person) who can assume responsiblity for the whole job. It will be espensive but that's your only assurance that the job will be done right.