Save your cash
I have a w203 52 plate, had it from new, was back at Mercedes within first 3 years, has had 2 full re-sprays by 2 different body shops, Mercedes lifted the bill. Every year rust comes back on all the places you mention, I just rub it down and coat with hammerite and the use touch up to blend in colour, not perfect however keeps it at bay. Normally I would have renewed my car after 4 years however the thought of a car salesman pointing all the flaws in paintwork would drive me nuts, so this is a personal protest to Mercedes for producing cars with duff paint jobs and from reading this forum they are still at it. My car is immaculate every where else and has only covered 51k miles and I enjoy driving it however I don't think I will buy another Mercedes for this reason. My car is not really worth anything in monetary valve, yours will be much the same, I certainly would advise you not to waste any money as the rust will certainly reappear. Sorry I cannot offer a more positive reply.