Using an electronic "sniffer" is pretty standard procedure these days. Dye is still used for those slow, slow, slow or intermittant leaks that never are there when it's at the shop. A shop with the right equipment and expertise can swedge/crimp/press a custom hose for you far more inexpensively than buying premade from a dealer. If everything else checks out (recommendations, certifications, BBB, experience with MB's) then you may have a shop at the top of their game.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)