possible 300d purchase..

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hi, so i got to the 6 month mark of owning a car and am getting itchey feet. My current driver is a 2.5 190d and i found for sale a good looking 300d 3.0 litre 6 cylinder with low milage (compared with mine) it has done 148,000miles. the car is on ebay and im not going to have a chance to test it, but how do these 6 cylinder engines compare with my 5 cylinder in terms of reliability and fuel consumption? i imagine they are just as reliable, just wondered if there were any issues? also, he said upon cold start up it starts instantly, but it runs bumpy for a couple seconds then its ok. he said he though it was the "high idle control" i dont think he has a clue about diesels, i think its probably just a duff glow plug- what do you think? thanks alot guys!!!