"Upfront" and "up front" are a little different. "Upfront" is used as an adjective directly before a noun. Either can be used as you have as an adjective in the predicate, but "up front" is better here. See http://grammarist.com/usage/upfront-up-front/

"Ahead" is a direction, and "front" is a location. But in the sentences you used them they are both correct.

All these phrases are correct, it is really up to you and how you want to speak. I personally would say, "I see a car up front" but some people say, "I see a car up ahead of me" or "I see a car upfront" which are all correct.