I would agree that other non-body panel parts are generally way less expensive at the bone yard. That is because you don't have to pay for every rat-stinkin' bit of plastic/metal when you get a whole "chunk" (as you did in getting a front end). It can also be a lot less expensive to get bone yard mechanical repair parts like suspension parts (usually sold as an entire assembly) or electric parts (like electric AC compressor or electric vacuum brake booster). e-Bay has a couple 451 electric drivelines for $1000. Even with freight shipping that is less than M-B charges for one or two of the components. The deal includes the electric motor, charger, transmission, axles, and all the other electronic modules bolted onto the motor (is the DC-DC converter bolted onto the motor assembly??). If I had a place to store one, I would buy a complete driveline.
But I still like getting new, color coded body panels at the dealer. As you stated, you now have a red car with a white front end. First, I don't like painting plastic parts because adhesion and colorgloss match is unpredictable - even when you use the "correct" paints, additives, techniques, and preps. I was an auto painter for several years. And, paint, materials, and paint labor to paint the front end will likely be $500+ (retail pricing).
The drawback about used wrecking yard mechanical/electrical parts is that they are, well... used! They may last a long time or fail sooner. Given that these Smart electric drive cars usually have pretty low miles on them and the cars are mostly plastic should mean that used parts will tend to last a while and make economic sense. Also, since these cars are just driven locally, a breakdown when a used part fails is not catastrophic - other than the fact that you need a flatbed to fetch the things!