Thing is for me that E30 M3 Convertible is visually the most stunning but it is not well suited for a high performance package due to the structural rigidity or lack thereof. So actually my favorite engine which would be the 3.8L R6 S38B38 engine which they put in the E34 M5 would be misplaced because that should go into a closed roof car preferably without a sunroof.

That said a high displacement engine which has huge torque on the low end such as a 4.0 or 4.4 V8 would be my choice. You can cruise in that and use it as a glider but as the brits say it can shovel a bit when you kick it.

Alternatively an era specific 3.5L R6 would also work but then you run a super impressive shell on an engine that is really made for boats and 5 and 7 series 'barges'.

The one I saw being built had the 4.0 V8, quite a machine!