Floors almost always rust from the inside out, not the other way around. Unless you a planning to drive in the surf or salted winter roads any good paint will do the job. Undercoating is soft so it resists chipping when hit by debris thrown up by the tires. It's probably more forgiving with regard to surface prep than other paints. You can buy it at Lowes in an aerosol can for $7-$8. Judging from a lot of cars I've seen, undercoating was a dealer-added profit...I mean option.
POR-15 is expensive and unnecessary if there is no rust. What condition are the floors in?
POR15 will only adhere good to rusty metal or blasted metal, it doesn't like to "bite" to pre-painted surfaces all that well. If you will be brushing it on, a quart of "insert brand name here" of paint will be sufficient. POR15 does flatten out/flow very well when applied with a brush btw, a quart of the stuff will run you $40-$45 +tax. Rustoleum has a rusty metal primer also that you can get from Lowes/Home Depot at about $15 a quart, but it doesn't flatten out nearly as well. If your wanting something more than just paint, you can use Lizardskin which is a thermal & acoustical barrier coating, but its expensive to use, about $70 a gallon, requires special gun and would take a few gallons to do the underside. You can check my thread in the "Makin Progress" section if that tickles your fancy. Another option, but I shyed away from is to get some truck-bed liner for the underside. My previous experience with undercoatings has not been a pleasant one thus far. Just my 2 cents.
If you've gone as far as scraping the old coating off the bottom, you might remove the carpet and do that side while your at it.