Technically, you can wait as long as you want before deciding. It just means you would have to make the transom cut-out after the hull has been assembled, but that's not a problem. Also, if using a stern drive then you want a transom that has a consistent thickness so the motor will have a plain flat surface to mate against inside the boat. The outboard version of the transom has two 1-3/4" thick LVL motor boards on top of a 1/2" thick transom, total thickness of the transom through the motor boards is then 2-1/4". Below the motor boards, there are two layers of 3/4" plywood laminated on the boat, total thickness down there is only 2" thick. To build a transom that can go either way, just add a 1/4" thick layer of plywood below the motor boards to make the whole transom from top of upper motor board to the bottom the same consistent thickness (glass and fair to be flat) and rather than making the motor cut-out as for an outboard, just finish the top of the transom with a smooth arc from one side of the boat to the other. If you go with the stern drive, then the transom will be ready. If you go with an outboard (or two), then you'd go ahead and mark, then cut out, the cut-out for outboards.
See attached drawing for the outboard type transom. I can send you the stern drive transom drawing after I get home tonight. Otherwise, 'contacting me' means sending me the approximate weights and dimensions of stern drives that you are considering so I can pop them into the CG spreadsheet to verify the resulting CG is within an acceptable window, e.g. to make the boat trim properly when at rest.
Brian
