Yes, pretty involved. You would need to plan the job, work out what needs to be removed, all the bolts and gaskets that should be replaced.
This is the reason the time to look at this job is when the oil cooler seals need to be replaced, as the intake manifolds need to come off anyway.
To make this easy for me, I paid and independent to replace my oil cooler seals, but I arranged with them in advance about me buying some low mileage good condition intakes that I clean and refurbed, and handed to them as part of the job, so they could take my older intakes off and just refit clean ones.
Then I sold my older intakes once I'd cleaned them all up to get some cash back.
I think doing it this way was something like £350 for the newer ones from ebay, and then around £200 I think I sold them for. But I have got a spare thermostat out of the original one that I'm keeping on the shelf.
For sale are the manifolds that were removed from my car; a late 2011 W212 with the 265bhp engine. My car had done about 86k miles when they came off. They are in pretty decent condition. I've already given them a blast with 6 cans of carb/EGR cleaner (I had an oil cooler leak!), but you might...
forums.mbclub.co.uk
A commercial garage won't want you to take the manifolds away to clean them because that means they can't finish the job in a day and ties up their ramp (and money earning potential). Hence Uncle Buck did the whole job himself.
Benefits? Probably better air flow in the manifolds. I know the amount of gunk that can build up can cause the swirl flaps to jam and bend and potentially break. The swirl flaps are intended to cause more turbulence at low air intake speeds (low RPM) and thus create better air mixing. Stuck swirl flaps can also damage the swirl flap linkage and swirl flap motor (also pictured in my old for sale thread linked above).