The 123 is a very light car. It weighs little, if any, more than a Camaro.
When you swap ANY engine into the car, MB or otherwise, the statistical reliability is going to suffer from the engine change. Engine swaps involve adapting this and fabricating that with only one chance to test and engineer that piece correctly. With production automobiles, every piece is tested and worked out before the car is put in production. You can bet that there were problems of some sort with the prototype. Any engine swap must be considered as the prototype. The only way you can maintain good statistical reliability is leave the car as engineered and built.
Best of luck,