Yes, you can leave it in D when stopped, but I heard that if you are going to be stopped for more than 2 minutes, you should put it into neutral. Probably not really needed, but what have you got to lose?
Trans fluid should be changed every 30k miles, though it seems as though you have the 722.6 5-speed auto (every US model 97 and newer had it). This is supposed to be a lifetime fill, as in never be changed. But if you want to really be safe, I would have it changed every 60k miles, just for the extra peice of mind (BlackMercedes has this done on his cars). It's a good $200 for the service, because it's a special synthetic trans fluid for MB.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)