Try telling them what your value is and why, its your value that matters not theirs, you are entitled to be put back in the same position as before the accident, either repair the car to your satisfaction or find one of a similar condition and spec (No matter the price, thats their problem)and demand the full replacement price.
Is pretty hard to find a near 30yr old Merc with just 30k on the clock nowadays I think mate. Luckily seems like I'm keeping the car and getting some cash in pocket too, haggling over the amount currently, but the good thing is it's repairable quite easily.