Since no one else has chimed in I will throw in my .02. I wrote a big, long post and chucked it because it was too long. IMO 3.5 heads and cams would be a downgrade on a 5.6 (smaller valves, ports, more valve shrouding, less lift on the 3.5 cams). Stick with the 5.6 heads and intake. It would be really expensive to upgrade 3.5 heads to 5.6 specs and the combustion chamber would get bigger in the process negating the reason for using them. If your goal is more CR it gets VERY expensive. The US motors have a hugh 30cc dish in the piston to lower CR. You can't just drop in high CR pistons unless you could find OS ones and have the block bored because from the factory each piston is indivually sized to the bore. I have seen 5.6 engines with 4 different sized pistons. Nothing will be inexpensive to do.
I am toying with the idea of building custom headers using reworked Ford SB ones. You can buy several sets to cut up for far less than the tri-ys. True the time will be much more but I like doing that sort of thing. There are some "hotter" cams available but not sure a low compression engine would benefit much.
Even in it's stock form the 560SEL is a pretty quick car so a good, stock 5.6 would probably get you what you want with improved exhaust and FI. Now my new post is longer than the original

__________________
Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT