I find kelly is a good ballpark figure to work from. Since knowledge is power I also use nada and hemmings to get a more complete idea of the range of prices that may be reasonable.
Hemmings.com is useful, but have to pay for other than "good" vehicles prices. They show an 84 300D Good condition at 4,675 don't list 85 or 86.
nadaguides.com shows '85 300sD (no 300D listed) @ $5825 - $8900.
Of course prices vary by region, and kelly considers zip code.
Happy shopping and good luck!