Hey julesybugs.
It was 10 years after my house was built that I had to replace my front door. Over that time the outer bottoms of my door casings had severe water damage which made the threshold come loose. I had to install a new pre-hung door. Inspect your door casings and threshold and if they are in good shape then you can just replace the door.
Sometimes a door will put up a fight and be "gappy" in places or be reluctant to hit the striker properly. That is the benefit of putting in a pre-hung door. If your inspection says that you only need a door then go ahead and get it. Often times it lines up with no hassle.
While you have the door knobs off, see if the weather has tarnished them. If so then I might recommend picking up a new door knob. If your old door has a deadbolt, you will probably need to pick up a bit to drill a hole for the deadbolt. Most new doors do not have an extra hole for a deadbolt. Bring in your old house key and let us re-key your new door knob to your old key. To finish the project, bring in a color sample of your old door and let us mix you up a quart of paint.
Thanks for the question and welcome to the community.
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