Well for warm starting, it could be the overvolt relay, AKA OVP. This is a very common problem with Mercedes. The relay is only about $50 and very easy to install. Maybe this is where you should start. It solved my warm-engine starting problem.
Wait, do you mean that the engine does not even turn when you put the key in? If so, then it is not your OVP... could be your ignition switch.
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!)