Nick, Chris is right. If the AC works/cools at the high blower settings, it's not likely to have a leak. There's a solid-state blower speed controller that's separate from the push-button unit that's also worth investigating. However, my money's on the blower motor. As bearings and brushes wear, it gets difficult to start and/or sustain the blower with the low voltage of low settings.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD