The original poster never mentioned a quick fix, and only mentioned a faster than f/2.8 lens after advice given here. I don't think that advice is the best plan given what we know.
The original poster initially wanted a f/2.8 zoom and I believe that is a VERY valid option for her or him. Trouble focusing with a small sensor camera was mentioned as a MAJOR issue - a fast prime on a DSLR will only exacerbate that. You can fix a lot of things in post, but focus is not one of them.
The original poster also made no mention of the location for the low light scenes - for all we know it's under her or his bed. Yes it could be a public place without permit, but to assume so and base all your advice on such seriously handicaps said recommendation. In fact, if unpermitted public shooting is part of the plan, my first advice would likely be to reconsider such a location - if something goes wrong, any bad publicity involving filmmakers breaking the law hurts us all in the end. Legal locations can be found cheap and/or free. IMHO shooting unpermitted should only be a last resort option.
Prime lenses wide open are certainly not the best option if you need to shoot quickly - changing lenses and shooting additional takes because of blown focus is not the fastest method.
Lastly, renting lights will almost always be cheaper than buying fast lenses, unless said lenses are serious crap.
OP says he has critical low light/night time shots. Its my guess that a fast lens is more within his budget, time, and convenience than a light setup in a public space at night which most likely he'll be filming w/o permit so he just needs to steal the shot on location. Obviously if he has the budget for filming permits, can get a nice light setup in a poorly light/dark scene then he'd go that route. But seeing that he's looking for a fast lens and a quick fix, I'm guessing that he's asking for a fast lens and a quick fix.
