It is not unusual for the cap seal to get old and inflexible, and start to leak. The cap is constantly splashed with oil from the camshafts and while it is not under a lot of pressure, if the seal is not working the oil leaks out onto the valve cover.
The smoke at start up can be an indication that the valve seals are getting worn. Is this smoke at start up something that happened once after a long rest or period of not being used and has gone away, or is it a symptom of every cold start? If it happened once after a long period of not being used, and no longer occurs, I would not worry about it.
One way to check if there is a problem with blow by is to take the oil fill cap off while the engine is running and look at how much smoke and gasses come out. If there is a significant volume of gas escaping, or if getting the cap back on makes enough resistance so you have to account for it and force the cap back on, you might want to have a compression check done. If there is no noticeable emission of gasses you have nothing to worry about.
Hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)