A W140 ignition switch would be quite a challenging job for a DIYer! It's not really hard to do, but it's not really hard to mess up either. Don't confuse ignition switch replacement with tumbler replacement. Remember, the ignition switch is the first line of defense against theft! It's made to be a bit confusing intentionally. I've seen more than one not-so-experienced person have all kinds of trouble with this job. Proceed with caution.