I'm looking for the same thing for a similar project. The only viable radar sensors I've found are only sold to oem manufacturers (Bosch, Continental, TRW). Banner engineering has some sensors that have a range of 5m ($400) - 15m ($900) but that's not long enough for ACC at high speeds. On ebay there's an audi A7 radar sensor for like $400 but there's no data sheets so I would be lost. Any help on this would be much appreciated.