Dude, get the rail system if you're going video, and then just pick up a cheaper, vintage option for the lens.
I'm assuming you're using canon (if you're on the Nikon system, stop being silly and get the 35 f1.8) so check out these options for insanely less than the Rokinon 35 t1.5 (which is the one you'd actually want since it has the gears already, and a declicked aperture ring)
Olympus OM 35 f2 - probably 200 bucks
Olympus OM 28 f2 - probably 250 bucks
Canon EF 35 f2 - 250ish
Vivitar 28 f2 "Close Focus" (I used to have one, bought it from 1337 for 65$)
Nikkor 28 f2 - Probably 250 bucks
Nikkor 24 f2.8 - 100ish
Sigma 30mm f1.4 - 250ish
I mean, there's tons of options. You can to the Rokinon later if you want, but in the meantime, why not save money and go with both - only with a stop or 2 slower lens (boo hoo)