ahead of you. installing a "blank" door slab into a pre existing frame is quite a task. Reasons: you have to be very precise on the mortise cuts to make sure they are the same as the old door, same depth, same angle, and same place.
Any small deviation, then the door will not operate properly.
The big factore is that the house has settled and the frame is NOT square any more. You have to take this into consideration. I bet if you put a carpenter's square at the top 90 degree angles, they are not true. The frame has contorted to the houses movement and the door has adjusted. There is no way you can account for this when installing a square door into a distorted frame. Even so slight.
Installing the frame with the door makes the door square the frame square and a workable door. This belive it or not is much quicker and easier than cutting a blank. And it will work fine.
I do this on a "ongoing" time and I always sell the frame kit when doing any door. If a person wants a slab cut, then I will do it but I charge more for a slab cut than for a complete pre hung door with a frame.