replacing the entire door with the frame and threshold will in the long run give you a more function door and one that will work smoothly.
When you install a door blank into a door frame, you have alot of variables to consider. The frame has canted one way or another, the door has warped, the hinges are tweaked one way or another and you are asking someone to mortise out the hings plate grooves for the old hinges to mount in.
Many times the door will not shut properly and the door has to be shaved. Then when the house settles again, the door will not close again.
Less headaches route is to replace the door frame, door, threshold and all. This is sold in a kit with all together.
A door kit with a metal skinned door with two holes cut out for locks sell for about $200, I charge $120 to install it. you will need new door framing in most cases.
When talking to the salesman, know these facts: Look at your door from outside to inside. Notice the side with the hinges and notice which way it opens, either in or out. Take this information with you and you will get the correct door as you now have.