If can still find the plastic pieces that broke off, use JBWeld to re-attach them. Spread liberally all over the panel and the area where they originally attached. Very strong.
If not go to a wrecking yard. They often have door panels at prices that will save you a lot of money.
Be sure to replace the inner door panel moisture barrier plastic if torn or missing. Makes the interior quieter and keeps moisture off the door panel, otherwise you will replace it again. Hardware stores sell this plastic by the yard, look for 2 mil or so.
Keep us posted,
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)