So, as an update for those interested I purchased the following:

Contax 645 Body - Prism - Film back -- Your basic setup.
140mm f/2.8
30mm Arsat and an adapter from the Ukraine.

I spent $2,175 even just about, I think I did well considering most sell
north of $2,800-$3,000. I have a great warranty on everything except for
the Arsat, which is almost to be expected. Considering a skateboard
could fly at the lens it's at about the right price point, also it's a 18mm
equivalent which I'm actually use to using a 12mm equivalent when filming
skateboarding. It's a subtle difference you'd think but you can actually be a good
step further for the same framing thus ensuring...ahem, granting better safety.

Quite excited to test everything and proceed forward, perhaps by Friday
of next week.