i have owned and worked on enough benz's to know that id trade this in a heart beat, while your benz has low miles, its getting to the point where its gonna start costing you a fair amount with upkeep, the 4runner on the other hand isnt gonna be as needy, but yeah look things over, test drive it, the timing belt isnt a big deal, nor is the water pump. and personally i wouldnt touch the either untill the water pump starts to leak, origonal timing belts can last 200k + even though toyota says replace at 90k. even if it breaks so what nor harm no foul it wont hurt the motor, it just will stop running. put a new belt on it and your good to go.
C class MBs are a night mare to work on, i have worked as a tech for an independent german car shop/tuner and honestly i hated MBs after working on and owning a few, and the C class was at the top of my hate list for the MBs. i sell it while its still trouble free and you can honestly sell it as such. you can try to use the timing belt and water pump as a bargaining chip, but i wouldnt let it keep me from the trade if everything else checks out. -you can get a timing belt, water pump and new thermo along with all new belts kit for like $250 and it can be done in an afternoon in your garage, its not complicated at all. the only thing that would keep me from this trade is if the 4runner was not 4wd, if its a 2wd tell him to go kick rocks