Ye, the C class form 96(n-reg i beleive)...was from a new era of mercedes: under american ownership they beleived cost cutting was superior to quality, so the c-clas, although based heavily, i beleive, on the w124 used much cheaper materials and was assembled without the same perfectionist attention to detail that the w124 had under the german authority.
As it has been stated the w124 has superior build quality even in comparison to most other mercedes, perhaps only rivaled by the 123 and the 104 s-class of the nineties....so there is no difference between the 320 and the 300...both are german developed and produced engines so withstand the same w124 reputation lost post-1996/7, with the introduction of the new eclass, (w210? i think....)
i am currently looking for a c124 320(coupe w124)...and have read that what you get for your money now represents how well it has been maintained and looked after, and unfortunatly the temptation of ragging the 3.2 litre was too great to some owners that the well being of the car was severley deminished...some cars ive seen for sale at £2000 pounds look very similar to the ones for sale at £7000....however it is what is underneath that counts now...if you read the mbclub thread, 'disaster with the alfa' (or some title similar) you will see the potential fiasco that can be caused by giving into a cheaper car to buy, however ultimately a bank account-breaking car to run or own.
In simple...there shouldnt be much differennce between the 320 and 300 engines, however it is the treatment they have received that determines how well they are now, and ultimatly it means that every engine will be individual now, and check ups(for instance something like an AA mechanical check up), are essential when considering buying any old car now
Having said this....as i said i am lookng to buy a 320 c124...and have seen examples with 100k+ miles on the clock from independant specialists....and the i have seen one example with 59k miles on the clock from a private seller...does anyone think that the lower mileage one, without a warranty is worth a shout, as it has less miles...or is it better to buy a higher mileage one with a warranty...i beleive that 60k miles is the fruit of the engine...i imagine the warrantied examples are the best option, however when there is a price difference of £4000 to £7000 i really am not sure..if anyone can shed some knowledge id be much appreciative..
regards