In general I agree with Glyph. Agility is your key ability. I would soft cap it (meaning take it to one below maximum) and then get some Muscle Toner- level 2 if you are staying with a 12 availability and level 4 if you are going to get Restricted Gear quality (and GM approval). If the most important thing about this character is being a gun fighter. I would suggest taking pistol (6) and a specialization in semi-automatic. Figure in a smart link this puts you at 17 dice with rating 2 toner and 19 dice with rating 4 toner. Depending on the table you play at that is somewhere between solid and OMG good. I would take some wired reflexes, a decent reaction, and maybe some Reaction Enhancers as well. I also suggest taking gymnastics rather than dodge for defense. Gymnastics can be bonused with a bioware mod called Synthacardium. Level 3 cost 30,000, or 6 bp, for +3 to all athletics skills. That is half the cost of the same amount of dodge skill. It works just as well for ranged defense. It is somewhat inferior to dodge for melee defense, but but if you are going to take a melee skill that will cover that fine.
As far as legwork, that could be several things. As Glyph said you could just buy alot of contacts. You could also take a decent charisma, some Etiquette(Street) and some Intimidation (physical) and you have a street smart guy who knows how to lean on someone. If you want an implant in this area then go with Tailored Pheromones. It bonuses social skills (or is it directly to chr? I would have to look it up)
The only thing that I might disagree with Glyph on is the cybereyes. I used to slap cybereyes on every character I made, but I have modified my thought on that somewhat. Unless you plan on cramming every goodie and gizmo possible into the cybereyes, you can get the same thing in a good set of goggles for less money and no essence lost. Not saying cybereyes are a terrible idea, but there are alternatives, especially if you are short on cash or essence once you get all the other stuff I suggested.
All of this is alot of chrome and custom work, more than a typical ganger would have, so you need some kind of back story for where he got the money, or who did the work for him.
Edit: change to automatics rather than pistols. I would suggest submachine guns rather than machine pistols for a specialization, as the machine pistols in Shadowrun are not among the best weapons.