I don't think there is anything on the market which will achieve that goal.  You are asking too much when you want to pick a row (+/- 10") from far enough ahead to be able to turn in and be on the row within about 20 feet.    Count rows, turn the end of the boom over a blob of foam, or accept that it is going to take 50' out into the field to decide which row you want to be on from GPS, and still be 50/50 whether you are right or off by one. 

Not trying to rain on the parade, but someone with no experience with guidance equipment who is going in with these expectations is going to be disappointed no matter what equipment he gets.