460 280 vs 463 350 td
the 350 turbo diesel is a very problematic engine and best avoided, it will have more torque than the petrol so not much in it for ultimate speed but you will need to use the gears more on the 280
A W463 - 350TD will have more torque but that may not translate in being fast or quick, quicker maybe but from what I gauge of you, it will not do.
There were no 350TD in RHD only available in LHD and as our member prwales has stated these are a renowned problematic engine, blown head gaskets, bend rods etc, etc...
Fit a turbo to your 280, PRWales will sell you a nice kit he has been going to fit for the last ten years .
IanA2 has it now but it will only fit the m103 engine, possibly the early 24v 3 litre m104 but not the 280 m110, nitrous is your best option
Deleted by Pistonhead - subject now now irrelevant.
A 500 would sort your issue :-)
If your engine is tired but the rest of your G is servicable, perhaps your best course of action without spending much money is to source a M110 engine from a contemporary E or S class. The hi-comp is good for 185 BHP and it will surely pull better and consume less than your current engine.