Installation options are like this:

Insert or Pocket - pull either the interior or exterior stops, drop in the replacement window (i.e. frame and sashes as one unit), replace the stops and/or cap and wrap the trim.

Sash Packs - Pull a stop and pull the jamb liners, insert new jamb liners with sashes. Will look similar to current wood window but not the greatest when it comes to sealing up well if the opening is out of square.

Full Tear out - rip out everything (exterior and interior trim) down to the studs and install new, complete unit. New interior casing, new trim, etc.

Thank you WW
I am having a problem finding anyone who will do a tear out.
I suppose I'll just get the insert and stop worrying about it.

If the windows are in okay shape, most windows can be made to work well as inserts.