Been there, done that on vehicles I've owned for the last 25 years!
High-pressure, low pressure, engine cleaners, Simple green, you name it!
What I've learned from experience:
Most high pressure sprayers at the self-service bays have a high/low trigger. Use it! Remove bugs and debris from the radiator fins with high pressure, grease and grime from the lower engine surfaces (where most the really grimy areas reside), and low pressure on everything else, (painted surfaces, upper engine areas, etc.)
MB isolates the sensitive electricals from moisture in obvious ways for a reason...so don't go hosing down the battery compartment area. Also stay away from ignition components...wont do much real damage, but your car will run like $#!# until the moisture is removed.
Pay attention to areas where water can "pool". Swab off the excess with a towel as best you can. Spark plug receptacles are particularly vunerable in this aspect, so use caution.
Hosing is best done with the engine running, but watch out for moving engine parts (pulleys, belts, fan, etc.) especially when you are probing around with the hose or towel or HANDS!
If you choose to do this with the engine off , DO NOT WASH A HOT ENGINE!!! A warm engine is best to assist in grime removal.
Some engine cleaners are flammable (go figure
) so be especially careful using these. Others are caustic enough to remove paint or discolor unprotected metal surfaces.
I perform engine washing maybe once a year, to assist with finding a newly developed leak, or some other potential problem. For a daily driver, dust and dirt accumulation occurs too rapidly to do this for cosmetic purposes.