I agree...your money could be better spent elsewhere than trying to make your ML look like a 55...
...as far as cost, consider that when I broke the instrument cluster lens on my SL, I was out $327 at the dealer parts counter!
And I'm just talking about the plastic cover, not the actual gauges!!! SO that's not a bad price compared to replacement parts costs of the other MB models...
...as far as coding, I'm not sure, but the ML55 might have a different gear ratio profile than the ML430, so the speedo might have to be recalibrated...