The 255/70/18 is what is on the Sahara, stock. So, yes that size tire will fit. You will obviously need to buy new rims because a 18" tire will not fit on a 16" rim. You may be able to get a tire/wheel package (off a Sahara) on Craig's List, that will solve your problems there. Many are usually avail from owners buying aftermarket rims. They will usually throw in the stock Sahara tires also.
A 275/70/18 will also fit, with minor rubbing (front) at full turn. Can easily be fixed by adding washers to the turn stops (so that you can't turn quite so far) or adding wheel spacers for ~$99 per axle.
My wife has a '12 Sahara, that's why I know.
We will go with 275/70/18 (on stock 18" Sahara wheels) with a 3/4" leveler spacer up front (to level out the vehicle some-it's about 1" higher in the rear).
Will add wheel spacers (as small as I can safely add) also. Currently the thinnest I've found is 1 1/4".