The points from killing someone are in the minigame creator. You can change this around however you like, but the default amount is 1 point per kill.
Then, to buy things with those points:
OnActivate - Self - VCERetroCheck - IfPlayerScore >= (Points needed)
OnVariableTrue - Player - Additem - (Weapon)
Then, you can add stuff for when you don't have enough points like:
OnVariableFalse - Client - Centerprint - (You do not have enough points to buy this item!)
You might also have some other event after "Self" instead of "VCERetroCheck" called "ifPlayerScore". That works just as well.
Not sure how you would do killstreaks though.