4-door shells tend to be stiffer than 2-door, although there are two extra bloody great holes in the side, this is more than made up for by the extra bracing that is added around the door frame to hold it all in place. The down side is that this then results in extra weight (especially in an E30), which you can't really get rid of.
The other thing I would suggest you consider when looking at which shell to use would be the length of the wheel-base and the ratio to the track width. Can make a difference in terms of turn-in, under/over steer, etc. and give a better 'starting point' to adjust from.
If you are really worried about the twisting noises Michael I suggest ripping out all the interior panels and adding a full cage, which runs to the strut towers front and rear !!