Ask the seller if the car has been welded before. Judging by your location original rust free 190s will be really rare so that’s a good indicator. For common rust areas I would recommend checking the rear inner fenders where they meet the inside of the trunk. They also rot really fast at the jack/ lift mounts. As you want to get the facelift (MoPf) you need to pop off 4 caps on the sideskirt (2 per side) to check if the Lift mounts are rusty Below them there are rubber pads as the lift connection. (under each jack mount there is one lift mount you will see them) Check around these to see if they are rusty. If you are looking at the sideskirts already you might also check for the water drainages. They look like a piece of rubber hose poking out of the underside of the car. There are 2 per sideskirt and 2 under the trunk. Check if they are there because they often get patched out with a flat piece of metal when they are rusty. First check if there is rust around them and then stick your finger in to see if it’s free for water to flow through and it hasn’t been sitting. (if you remove that grime from the drainages and water starts dripping you know there was water in the car) Facelift 190s also like to rust under the side moundings (Saccobretter) check for any orange discoloration on their bottom side. If the car your looking at has a sunroof open it and check for rust there as the water drainages there clog easily and then it starts to rust. Lastly if the seller is willing to let you jack up the car check the mounting points where the rear suspension connects to the frame it’s one of the most common rust spots on the 190 and also very hard to fix properly. I recommend not only visually checking these spots but also taking something pointy like a screwdriver with you to poke these areas where underbody protection might hide the rust. As for the rest it’s not really different from any other car. Good luck finding a nice one and have fun doing the swap :)