Tunnie - remember the trade-in price when he's knackered it will be far, far, far lower than what your are currently showing for 4yr old Mercs etc. Assume all the dealers you are checking against are like the Roger Buddens of this world, looking at betwwn 30-40% markup on what they give you. Thus I don't think ETA's figures are that far off TBH.
Remember, that home servicing will also cause heavy depreciation, rightly or wrongly.
If depreciation is a factor, you need to be going with BMW or Audi etc, keeping the FdSH up. Merc aren't in the same league, as although they have a loyal following, they are not a desired car by most.
He would get the lower ppm figures, as effectively its a company car, as he gets a car allowance. To get the allowance and the higher figure would end up with a tax nightmare. As this allowance pays for the running of a suitable car, the ppm is only to cover fuel. Which it would in a 2.2 auto Omega.
Another option is to use the allowance to lease a car. That way its bought, managed and serviced, and often insured by the fleet company. And ppm covers fuel. Oviously, unless his company is very generous on the allowance, he's be in shitbox territory (Astra/Mundano/1 series etc)