5-8 seconds for pencil plugs is plenty at that temperature. I recommend checking and replacing the bad glow plugs, and give the car a valve adjustment if you haven't preformed one in the past year. That will greatly help your cold starting ability.

EDIT: whats the condition of your fuel injectors, poor spray pattern and unbalanced pop pressures will cause rough cold idle.

__________________
1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg