Thinking about this, The shell swap is not the best idea actually....
Reasons are as follow.
Firstly to find a shell, you need to find an undamaged car, so your looking for a 118 or 116 for cheap enough... that may be possible.
But now if you do a full transpant of engine, motor, harness ect ect,your going to spend more in labour to do it properly.
Then you have the problem as well of a change in chassis number, which will be a ball ache, followed by a perm entry on the cars history in the form of a police clearance, this essentially then becomes a code 3.... (Cois, help me out here on the code 3 issue)
However, what your looking for is an accident damaged vehicle, where you can purchase the entire side from. You will get away with about 25k in parts, maybe a bit more. Lets be silly and say 50k (double my parts car estimate) far less then a second hand 116 that will require police clearance.
I found one yesterday that would have been perfect, except its an E87, 2 door with a roof. I could have picked up the left hand side of that car for 15k without a door and bonnet, but that included a dome.