Steve, First, How good is the door frame, if its ok, then dont mess with it.you could be giving yourself more work than you need to. Look around your timber retail store you can find the door that will match the rest of the house ( almost) your problem will be the size. 2.6" x 6'6"/2'8" x 6.8" 2'9"x6'9", I know this because all doors are standard where I live. and most of the doors we have come from Canada or the U.S of A. If your old door is an odd size then get the door that is almost to the size you require, you can always take a bit off top/bottom, or sides, ( sides not to much) remember you still have to fit the lock. Second: Most outside back doors swing out (away from the house) this is for fire hassard.but if you only want to change the side the door swings on then no problem, hang it on the right/left ok. but if you want to which the way the door is opening then talk to your planning department if its ok to do this(and get in writing) I dont know what type of frome you have, but if it is just a frome screwed to the wall then no problem. if you have a very old house some times the top transome ( the bit on the top) has ears which are built into the wall. If this is the case then look to see if there are no power cables running up the inside of the frame ( you never know) get a hacksaw blade and cut between the frame and wall cutting off the ears and leave them in the wall, again get a frame that is the same size wide, the lenght can be cut to size. When you fix the frame remember you have to fit hinges so think where you are going to drill and fix the plugs, and away you go.