There is no real answer to your question. Just more questions.. It all depends on how adept you are as a DYI'er. If you don't really know your way around tools, engines, hell all things mechanical, you might be best served, paying someone to do the job..
You're into unchartered waters here - because 400/420 starters are so durable - very few failures to date. Just about everything is tight on these cars - but specifically what's in the way? Can you post a picture?
My E420 has had a major poop down. When I started the car and pressed on the brake my ABS and ASR light popped on. Also all of my gauges went dead plus my brake lights went dead. Help Please!!!