The majority of the C300's out there that are owned by individuals are EF mount. I've only had one call looking for a PL mount version. I told them to call a rental house. There isn't a difference between it and the PL version besides the mount(and the differences associated with that-no communication with lenses, etc). If you go PL, you are only going to be able to use PL, you can not put EF on it. PL has a deeper flange depth than EF. In theory, you can put PL on EF(because EF is shallower), but there are only two adapters that I've seen and the reviews and results are mixed. Not all PL lenses are the same and therefore not all will work with these adapters. One of the biggest problems I've read about is that some of the lenses can't achieve infinity focus when used with these adapters. You could work around that if you're focusing yourself visually through the VF or monitor, but you can't pull focus by the witness marks. It's pretty much a case-by-case basis, is what I've been told by my rep at Abel(I don't think they sell any, but they did some checking for me).
As far as 2/3" lenses, there are a few adapters out there. I believe it's MTF that makes one that has a small glass element in it and the B4 lens MUST have an extender(which must be engaged) to work to blow the image up to cover the s35 sensor. IBE makes one that is sold and branded by Abel Cine. It contains all the optical elements to blow the image circle up to cover the s35 sensor, BUT it will cost you almost 2.5 stops(the MTF will, as well). And the IBE has user swappable camera side mounts in EF, PL and I believe Nikon. They're nice tools in certain situations, but there is degradation in the image quality. And you will have to use rails and a support for the ENG lens.
As far as still zoom lenses not being parfocal, I don't think it's a hard and fast rule, but most probably aren't. "Live zooming" has never been something still photographers have had to worry about, and therefore probably never part of most lens designs(probably makes them less expensive to design and make and lighter weight).
There is no doubt that the C300 makes a beautiful picture and if you can get one at a good price and recoup the cost quickly, it wouldn't be a bad investment, but if I was buying new right now and looking down the road, I'd be leaning towards the F5/55. It can do more and you're not locked into one lens mount. HD, 2K, 4K, HFR, better ergonomics, native/better VF's, almost universal lens acceptance, better/more recording codecs, F/55 has global shutter and on-board RAW recording.
In my opinion, even as a C300 owner, the F55 is the best/most versatile damn camera for the money on the market. I don't have any regrets buying my 300 last year, but if I had to do it over again in 2014, I can't say that I would.
