Engine swap: rebuilt vs. used-Need Advice

A couple of months ago i fractured my second crankshaft, and my mechanic tells me it's a defect in the engine, so he says i need to replace it. Yesterday, he gives me two options: a used engine with only 100,000 miles on it for $2975, or a newly rebuilt engine (that he did himself) for $5200. The car is an 84 300D in very good condition. I really love the car and it's one that i'd like to drive for many years into the future, so i'm kinda leaning towards the rebuild. So i'd like to know what you guys think about this, and whether or not this guys prices seem reasonable. He's a very reputable mechanic in the area, but he is the only mercedes mechanic besides the dealership in a 100 mile radius. I'd greatly appreciate any advice you guys could offer. Thanks