Love the pictures of the cars....brings back some memories. I currently have a 2002 V8 X5, but had a few of those E21 3 series cars, including a race car.
You didn't post pics of the interior of the 3 series other than the radio, but in the US, there was an "S" package for that car, that came with a sport steering wheel and very nice Recaro seats. It either came in black vinyl, or a very nice dark grey cloth. You'd have to find an entire interior including door cards and rear seats to do it right, but that's an awesome, and very period correct upgrade for an E21 3 series. I'd hold out for the much nicer grey cloth.
Also, if you're motivated for a bit more power in the 323i, the bottom end from the American E30 325e will mate up to your 2.3L cylinder head with little effort, bumping displacement up to 2.8L. I have seen a number of these combinations, and it can work very well....and will look 100% stock.