Read this first - lots of info, applies to 1987 300D/TD, power is one of the issues addressed:
http://www.w124performance.com/docs/mb/articles/124.1x3_buying_tips.txt
0-60 time should be in the 10-12 second range. Slower than that, start tracking down the assorted possibilities... filters, IP linkage, ALDA signal line blocked, low/no boost, etc, etc...
