A very complex question. I can't answer it. You will find guys that are successful not because of their individual ratings, but because of the blend of their ratings. The best advice is to look around the world, and I like the advice of using the franchise rankings page to "get an idea" of average ratings. Start there. Then keep looking around...when I first started I would go into the trade page to look up not only the big league rosters of every other team in the league, but also every minor league player they had as well. I wasn't making offers, just looking. I spent many hours doing this. Gradually I came to the point where I could glance at any players card and their current ratings and just know what they could contribute. I don't have any hard specifics on how to do this, just keep looking and it will happen.
Once that happens, then you move on to more looking around and now you pay attention to projected ratings. Sooner or later, with enough looking around, you just know if they will reach them or not. You just know what players are worth, and you just know if they will help or hinder your team, now or in the future.
More than likely this reply only confused you further. That is a good thing, start looking around.