i would say around 70 to 80 points range in each catigory will provide a win most likly.
BUT you will need to tune the car for optimal performance, meaning the suspention, select the right tires, the right LSD settings and brake setup (downforce most likly).
sometimes you will want to add some small HP mods, but i havent needed to do that very often at all.
i would say that MOST bspec drivers wont need to go much further than gaining an avarage of 80 points per skill to win the game without problems, or the need to ever raise the level further than that.
