Are you sure it's a 603 (124.130)? No chance it's a 606 (124.131)?

The 124.130 is geared lower in first than a 124.133 (US 300D turbo), but I don't know how a torque converter influences things. I doubt a turbo will help you up a boat ramp since it's not like a truck turbo that kicks in right above idle. Or I don't know how to get a boat up a ramp The US 124.133 (automatic only) is geared about the same in fourth (top) gear as the .130 is in fifth when you consider the final drive ratio (2.65 vs 3.46). The more telling difference might be the torque specs of 137 lbft at 2800rpm for the .130 vs 195 lbft at 2400rpm for the .133.

I don't know that the naturally aspirated 603 is a must have feature in and of itself. If you think the turbo 603 intake manifold gets in the way of servicing glow plugs and fuel system, wait until you take a wrench to the naturally aspirated engine. It's the 5-speed and Euro touches that will sell you on the .130/.131. Put another way, I doubt you'd choose a naturally aspirated 5-speed 300D over a turbo 5-speed 300D (I don't know if MB made such a thing) all else being equal.

Sixto
87 300D