RS-485 is not very compatible with i2c. There are some chips that split the signals and that would allow 485 to be used but that's not very suitable for what you are doing IMO.
The P82B715PN is probably your best bet, either that or ditch i2c and use async serial. That would mean having a small processor at each sensor though.
How much distance can I can expect by using this IC?
Don't know, from what I've seen they never quote distances in the docs for these i2c extender chips, you need someone who has tried it.
Rob