Buy a manual Merc is financial suicide. You will never be able to sell it unless you take it to Spain and fit a meter.
There is a limited market for such things , especially once the car is older/higher mileage .
I bought a late , Indian assembled manual W124 E250D for my girlfriend because she specifically didn't want auto . As it turned out , she felt the car was too big and replaced it with a diesel Golf .
With the manual box , that W124 returned 50mpg+ and I had no trouble selling it on to a member of this forum ; he later exported it to Hong Kong where it presumably lives on .
Once the car is , say , 10 years old with 150K + on the clock , there won't be a lot of money in it anyway and the auto/manual part of the equation won't play a big part . There are still plenty of ( misguided ) people in the UK who don't like automatics , so it won't be that hard to sell .