The earlier SOHC M103 (2.6/3.0) is prized precisely because it predates the biodegradable-harness era and is considered trouble-free, easy to service, and DIY-friendly. Owners cite very high mileages on routine maintenance, and some prefer the M103/M104 six specifically for ease of ownership.
M103 straight-six: no harness worry and near-bulletproof
Mercedes-Benz W124 E-Class · mechanical