As far as affordable PL mount lenses, Zeiss CP.2's are probably the most budget friendly option in line with the cost of L series lenses (but still quite a bit more expensive for a full set). But their mount is interchangeable and you can get them in EF mount, so no need for an adapter to use those lenses.
I believe Red Pro Primes can have their PL mount modified to fit a PL adapter on the C300.
***EDIT: I completely misread OP's post, please disregard!***
Holy crap, I totally misread! I hadn't even ever considered the reverse since the flange distance makes it impossible. Thanks for pointing out the discrepancy. Sorry for offending you.