There are several types of chassis dyno's out there - "Dynojet" being the most common. Another couple common ones are "Mustang" and "DynoDynamics". Both the latter tend to read lower than Dynojet; doesn't mean the car is making less power, just a characteristic of those dyno types. Another good base line to get while your car is stock, and a good supplement to dyno testing, is to run the car in the 1/4. Trap speed (don't worry about ET so much, that's more a driver dependant item
) are fairly consistent, all else being equal.
Re what gear to use, dyno operators like to use the gear that gives (or closest to) 1:1 ratio (has something to do with minimizing built in errors in the way HP is calculated) ,which in the SLK55 is actually 5th gear. But I'm sure your dyno operator will know best what works with his particular setup.
I'm also planning to get on the dyno next week if all goes to schedule.