Unless you sell it another car guy who likes to tinker, it'll usually lower the price. Anything aftermarket like that does. I replaced the seats in my Toyota with seats out of a Lexus. They're much more comfortable seats, they're leather, they match the car, they're power, and they reduced the resale price of my Toyota by about $3K. That's just how it works.The problem i have is i have posted it with a fairly standard price for a SLC in Australia and would like to know if you knowledgeable people from BW think that a upgraded ECU+Fuel+spark system is worth more then a standard SLC or if it drops the value?
If it idles as smooth as you've described, though, it might be a break-even. Especially if the potential buyer has test driven others while searching. Just play up that point, and downplay the scary "this isn't factory and a standard MB shop won't know how to work on it" angle.