Can you Initiate? Can you do Initiatory Ordeals? Does your GM allow PP in place of metamagic (it's an optional rule in SM)? Does your GM rule that Improved Attribute (Reaction) does not stack with Increased Reflexes?
Far and away best purchase you could make would actually be to get more money, install 1, maybe 2 points of bioware, and buy your magic back up. All4BigGuns will probably be by in a minute to explain how if you do that, you deserve to die of ebola and cancer.
If you don't want bioware, your first best purchase is definitely a sixth point of magic.
If you are allowed to use PP for metamagic (you seem to be using enough other optional rules that your GM is probably OK with that), initiate as much as you can afford. Take Adept Centering for one metamagic and PP for the rest.
Assuming you do not want any ware, and that you are allowed to use initiatory ordeals, and that IA (reaction) and IR stack, that would be Magic 6, then 2 initiations. Select "Improved Combat Ability: Pistols" as your 3rd way discounted power, take 3 levels of that (1.5*.75=1.125 PP), and Improved Ability (Reaction) 1 (.75 PP). I really highly recommend going down to only 1 level of Attribute Boost: Agility, which would free up enough for an extra level of Combat Sense. Otherwise, sit on the last .125 PP.
Also, you might want to check my sig and the Gunslinger for a similar character; in particular, you can crib some useful modded pistols.