I think you reversed power and ammo there, or you meant throwing spears, Tears.
For build, 18/18 is good, with 6 PT and 3 HA, you can go 21/18 if you cut ground skills, but I prefer more rounded play.
For the weapons, I suggest just buying all of them eventually, and trying them out. Every throwing weapon throws differently, especially from horseback, and they are also good for different situations. Javs and their ilk throw easiest form horseback, I think, but really everything works.