Yeah, something like a cracked deck kills the value of the mower as a collectible.
While I love the D-series mowers I have managed to collect, the newer F-series engine is better (IMO), although less rare in good condition. The main bearings on the F- are needle bearings as opposed to bushings (D-series.) The F- also has a larger displacement.
If you keep your eyes open, I imagine you'll find some great deals.