Replacing wooden front door with existing conrete sill

[lworbey]

I recently moved house and want to replace my front door with a composite door. The existing door has a large concrete sill underneath and closes against a metal weather bar that sticks out of the concrete.

Should I order a door without a sill and sit the frame over the metal strip and fix down into the concrete sill or do I try to remove the concrete sill and then brick up back to floor level and fit a door with it's own sill? I can't seem to find any guides or information on how to deal with this and yet there must be plenty of houses that have this setup.

 

[lworbey]

I'm not sure if this makes it clearer but the image on the left shows how the door closes now. I could fix the new frame as shown on the right perhaps although it would make the threshold higher up or should I just take the whole concrete sill out and start again so I can bring the threshold down?