From what I understand polypropylene is not very good for keeping oxygen out.
Since its free, you might try it out, see what happens. If your beer tastes like cardboard you know not to use it for this.
If that's the case you have a good container for mixing/storing cleaners or sanitizers.
My guess for short term use its ok. I would not use it for long term ferments like lagers, wines or meads.
If you really want to go plastic, PET is the best route. Look at the soda bottles or the plastic beer bottles. They are all PET. You can ID the PET with a triangle shaped recycle symbol with a 1 in the center. You can also find a PET or PETE marking nearby. I have 4 PET 3 gal water bottles that make for excellent mini fermentors ($4.99 each w/ water). Its just a matter of time before we start seeing these in 5 gal sizes.
I would also cart your bottle into the LHBS and try fitting it with a carboy cap before using a plug.