There's no easy way to remove undercoating. Best shade tree method I've used is a propane torch to heat it/soften it, and a good old paint scraper. Keep a fire extinguisher handy and be prepared for a messy job like you've never seen.

On the replacement coating, it depends on what level car you are building. The OE show guys have a pretty complex process of replicating the factory undercoating both in material and spray pattern.