Well, a 240D (regardless of transmission) can be outrun by senior citizens in wheelchairs, so you're setting the bar pretty low. That pretty red 300D non-turbo will be quicker but not by a huge amount. You could swap in a turbo motor and THAT would be a big improvement.
Given that pricetag though, you'd be crazy to buy it, or any W123 for >$5k. Spend $5k on a pristine late model W124 (i.e., 1995 E300 or E420) and you'll have 10x the car for the same money. Trust me. I've owned several W123's and I hope to never own another. The fact that your original question inquired about "quickness" and "power" automatically makes you a prime candidate for W124 ownership.
