$6000 may not be enough for a Canadian dealer...they charge this amount for replacing the struts (and we ignore the little matter of 13% tax).

Now for struts/shocks I just realized we are talking SLS, thus chances are, is not RSS (only '97+ vehicles are CVRSS).
So if the SLS you are looking at has no F45 among the RPO codes then you have much less expensive passive struts available in OEM and aftermarket for your year. I would not be so concerned about the struts on a passive suspension vehicle, all 4 corners will cost less than 1 active corner.
Now if it is a (rare) F45 SLS, well in that case, remember the old bounce the vehicle to test the struts/shocks will not apply. With the vehicle not moving the valves are wide open thus the struts will feel very loose (like worn), you must drive it to tell how good or bad are. Vehicle should be soft and comfy driving straight on slow speed, and should be sharp, flat and less comfortable at higher speeds and sharp maneuvers (within the safety zone).