I just got a 5 gallon bucket from a local bee keeper. The honey fills up to the rim of the bucket. My normal primaries have head space, which prevents overflow on what little foam might be made. So, if nothing else, I think mine would be a 4 gallon primary.
The plastic on there says it is #2 (rather, the recycling code). There have been plenty of discussions and links regarding BPA release. It seems, in my esteemed judgment, that #1 and #2 are more acceptable than #3-7 or "Other", but it is still a personal decision.
In my view, I drink from plenty of those little plastic water bottles (I'm sorry environment) - and we've all gotta die of something. Me? I'd do it. In fact, if I can figure out how to get a rubber grommet or other means for an airlock, I just might. To me, it sounds like a primary fermenter that I already bought.
I'm thinking a hole the same size as the stem on an airlock, combined with a dab of aquarium/food grade silicone up at the top of the airlock and it might just work. Not 100%, mind you, but better than nothing.