Hiya 190D,
If you search for the alternator upgrade thread on this forum, it has a lot more details. But to answer a few of your questions: The 143A was used
only on 1996-1999 S and SL models with a V8 engine (420 or 500). I got it from a salvage yard. Physically it should just bolt right in place, although you will need to swap your pulley on (or buy a new one). The wiring must be upgraded, and the new alternator has a post instead of blade terminals so you'd need to change the wires anyway. You don't need to go as big as I did but I'd use 4ga minimum. I noticed no power or MPG loss, but the new unit weighs ~17 pounds compared to ~11 for the stock one. Let me know if you have any other questions...
Regards,