There are alot of threads about protecting your ability to claim under the 30year mobilo guarantee. Some people have succeeded in claiming without full main dealer service history. So, really, go to an approved bodyshop as Flash says. It will be cheaper. I was originally quoted about £1,400 by a main dealer to get a bonnet replaced and painted for a w202 but going direct to the approved bodyshop saved me about £700 . Mind they did want to spray the wings too so as the bonnet would blend in better from a paint matching point of view - so >£400 quoted here - whether this was worth it is another story - I mention it because they may say the same to you. So £400 seems reasonable to me.
At the end of the day, its the people who pay main dealer labour rates that 'pay' for mobilo claims in advance! I prefer to save money now and keep an eye out for rust. Note, it would be wise to ask the bodyshop to have a look around the car at the same time. I have no idea if yours is a w211 or the earlier w210 - if w210 then looking for rust is worthwhile.
My 5p
Rgds
Les