Shop around. I’ve negotiated with main dealers before, last minor service on an 2004 E55 AMG - £84+VAT. That was at Mercedes-Benz of Guildford. I supplied the oil, they supplied the filter.
Stamped book, winter fit screenwash, washed and hoovered, free coffee etc, full visual health check (with video report), they even did some software update at the time. Some dealers are competitive for older models, I had a discount on the labour of 40% and a 10% discount on parts too.
Before that I had a different E55 AMG serviced by Mercedes-Benz Brooklands, including them supplying oil, filter, screenwash etc and very nicely cleaned throughout plus stamped service book and Mobilo cover at that time - £139 inclusive of VAT.
If the car has low mileage and full history I don’t see the point in losing that. You can always do the additional work and repairs yourself if you’re keen?
When it comes to resale, the car either has a fully stamped main dealer service history or it does not. It may be twelve years old but it is an AMG and I think DIY servicing as competent as may be will put some buyers off and/or affect the resale value. If nothing else it’s nice having a second pair of eyes over your car up on a ramp, they know what to look for and I personally find the video report these days useful for identifying issues you (or an MOT tester) may not otherwise have noticed
