I think it is a bad plan for you to look for a used door.
As woodnthings said, you will have to work around the lockset. So when looking for a used door you really need one with the correct swing so that the backplaned edge is going the right way. If it is going the wrong way, by the time you re-plane the edge the the right direction , the lockset hole will be 1/4" too close to the edge of the door. By the time you do the hinge side, assuming the original door was even backplaned, the total width of the door will have shrunk up by 1/2 and inch.
If someone had scribed the old door - or your current door - you will shrink it up even more.
As someone who has built hundreds of doors from scratch and hung many hundreds more, I would know what to look for in an old door before buying it , and also know when the juice just ain't worth the squeezing.
To make it way, way easier on yourself, I would try to find one that has neither been bored nor mortised for hardware (ie. a new one that you like). Then you can scrilbe it in so it will fit your opening, mortise the hinges and hang the door. Then re-check it, and when you are satisfied, bore and mortise for the other hardware.
That way, if you screw up, or if it takes longer than you thought (which it will if you've not done this before) you can re-hang the old door and secure the house.
If you don't, and you are married, you will probably be outside in your pj's twice an hour all night long checking on every stick and leaf your wife heard falling or rustling around in the yard.
Last edited by clampman; 10-19-2011 at 05:30 PM.