Oh don't worry Yaro, i'm not taking it personally (at least after i've had morning caffiene anyway) debate is always good.
The mercedes especially from the mid 90's are very bad for rust, i got lucky with my CLK and got one without tin worm thrown in for the price, but many have from that era have the wheel arches falling out. When GM took over Mercedes the Quality went south.
Any hole in body work from an LPG filler to an antenna should not be left untreated otherwise, yes you are correct it will corrode. I think that is something that separates a quality job.
I do take your point that yes it can be done for cheaper in the EU, many things can, however the converse to that is that i dont speak the local language, would have to add in the cost of a round trip to, say Poland, not have the warranty or support from a local (ish) installer, nor would have the paperwork in order to satisfy the DVLA or insurance company.
Most installers in the UK wont warranty or certify someone else's work and if they did i would be very dubious of their integrity.
An LPG installation should be an investment into the vehicle and without the verification of the install and the correct paperwork (for the UK) it's really just money thrown away.
Like i said the upshot is Caveat Emptor (Buyer Beware)