W126 ignition's can be a pita as I found out with the SD. So I take no chances and cough up the $20 for the official dealer cut steel keys. I really can't complain, the keys for a lot of my friends newer MB's cost between $150-$250 each!
Price out a key for an 05 CLK, or an S430!!
I have never had a problem with the heads. I have used the currant one on the SDL for close to three years now and the head is only showing slight wear. The key for the SD is only a year old and still looks new. Actualy I have a bigger problem with rust. Since this things are steel the rust like crazy in or around salt water, so I tend to keep a super light coat of oil on it. Don't want to attract dirt.