1. Not sure about why the glow plug harness needs replacement...

2. Glow plugs, $25/each, manifold gasket, etc. say $300 in parts.
$1400 @ $100/hour = 14 hours shop labor (almost 2 days) sounds a bit steep!

OM606 (your engine) has a reputation for 'difficult' glow plugs. Some change 'em all, some say "If it ain't broke, don't fix it."

Changing the GPs yourself is a bit tedious at best, and can escalate to complete "OMG! What was I thinking????" status if just one of them decides to be stubborn and stick in the aluminum head.

Others will add to this.