Most modern monitors, including the KRK's, will do the job. The key is to "learn your monitors", meaning learn how their frequency response is. Are they bass heavy compared to most standard car stereos? Do they give a clear response on the high-end? All of this comes with experience with your monitors.
Sure, it's preferable to have acoustic treatment, but for a lot of people that's not possible or necessary.
My advice: pick ones you like in the price range that you can afford (like the rockits). Search around for the best deal. Get them. Get a decent audio interface, too.