first, I would leave the 7 inch rise. this will keep out water and other problems. build up steps or ramp to smoothly enter and rise up to the door threshold.
What you have to do is to expose a section of the wall to get in there and build your headers, kings and cripples for the support of the door.
Make sure there is No electricity wiring in the wall and also plumbing. Many times this happens.
Instead of going into a long detailed explanation of the anatomy of a proper door way, I would suggest that you read up on that area of carpentry. A poorly designed frame for a door will only cause many headaches down the road.
But this project is not difficult at all. You will need a recip saw, circ saw, level, material, and that should do the big stuff.
Determine if you want the door to swing in or out and where the hinges and door knob will be located. I would definately get a metal skinned door with insulation in the middle. Will last a long time if kept painted.