I'm not too sure why you're trying to save money here, the cost of a head gasket is nothing compared to the cost of labour involved. It will probably be £30ish for an OEM part.
Gee you are a regular comedian arent you???
Help like yours I can do without. I thought there were people who dealt in discount parts...obviously a concept beyond your comprehension.
In this case the advice to get the genuine Mercedes parts (the head gasket in particular) is based on lots of experience. I had a very knowledgeable (20+years) MB tech replace my cylinder head gasket a few years back as a "homer". He was adamant that the only one to use was the genuine MB part. Also make sure the £350 quoted includes a strip and rebuild of the head plus valve gear/camshaft etc if the head needs to be totally stripped and planed/welded and not just a straight "head off head on" job.