Are screws an option? I always prefer screwing over nailing.
Not if he wants to get the job done fast enough that all the siding still matches, lol..
OP, consider a gun specifically designed for cable coil nails. DeWalt also makes one.
Don't rent, but strategize to minimize the cost. Search for discounted gift cards: it's usually pretty easy to find 10% off, then look for 10% off purchase coupons on eBay. Stacking discounts takes a little extra time, but worth the savings. Of course, it works for the nails, too.
If you're going to rent anything, consider scaffolding to make the job easier. Shop like hell for all the materials. When we did it, persistence paid off. Enough to buy a new nail gun and then some.