Hi All
I have a merc C250 , will it run on biodiesel ok .
or is there anything I need to pay attention to....

It'll run on well-made proper BioDiesel, but keep an eye on the IP for leaks....

Common to see leaks on the IP at the age of the C250 no matter what fuel...

I have a C250D and it runs like a dream on biodiesel, better than dinodiesel.
However there a couple of things to watch out for. The first is fuel filter changes. The fuel filter is mounted on the cylinder head and runs at about 60 degrees C when the engine is hot so you will not have problems of cold plugging. When you change the fuel filter make sure you also inspect and clean the gauze pre filter in the filter head where the fuel hose enters. This can clog up very easily even though the main filter is clean.
The second issue is 0 rings in the fuel system. There are about a dozen of them and it is quite likely that some of the original Viton ones have been replaced over the years with new cheap rubber ones. If you are lucky they will simply leak fuel and be easy to detect and fix. More likely they will let air into the fuel system, causing difficult starting. The only solution is to check and replace every one including the one hidden in the back of the injector pump on the stop switch.
I recently removed the cylinder head from a 250 that had done 60,000 miles on B100. The cylinder head was spotless, no coking whatsoever.