For any physical combat agility is absolutly key. I would consider softcaping it (meaning taking it at your metatype max-1), getting a quality called Restricted Gear (this is in Runner's companion, costs 5 BP, can be taken up to3 times, and lets you get an item of up to availablity 20.) and taking Muscle Toner 4 under bioware. This will give you an agility of 9 on average, depending on the metatype you are playing.
If you are going in on sniper rifle the skill is longarms, and you can specialize in sniper rifles. Longarms 6(sniper rifle+2) costs 26 build points. With a smartllink you have 19 dice with a sniper rilfe. In many circumstances it would be reasonable to take aim and do a called shot at -4 to your dice pool with +4 to damage value. That leaves you with 15 dices and a weapon that is either 11 or 12 P -3 AP.
I don't know the guns in the Aresenal book off the top of my head, but there are two Sniper Rifles in SR4A. The Walther MA-2100 has one lower damage value, and can not be broken down in a suit case, but doesn't have fussy issues in close combat and doesn't require another restricted gear quality to buy.
The Ranger Arms SM-4 hits a little harder and can be broken down into a case, but it has an availablity of 16F, meaning its another Restricted Gear quality at char gen if you want it. It is also more expensive. On of the downside of sniper rifles is that they are rated F instead of R, so you can't get a permit, even a fake one, for them, and unless you have a briefcase model they can be hard to move around in secure areas. Of course same applies to assault cannons, however, perhaps even more so.
Supporting skills will likely be things like Infiltration (the basic stealth skill in 4th ed) and some way to fight in medium to close range. One of the nice things about a very high agility is that with even moderate weapon skills you can end up with a fair dice pool on things, even if its not "the best" with minimal skill points. Try to get a decent reaction for defence, and maybe dodge. If you stay away from melee skills you don't have to worry about much strength, or if you grab a melee skill go unarmed and get a set of shock gloves. Perception is useful for all characters.
Depending on character backstory, if he learned to sniple in the military you would probably have some outdoor skills, but talk to the GM (are you the GM?) about how often this will be useful. In some campaigns you need a survival/tracking/navigation guy, in others you never see the city limits of Seattle. Make sure if he drops some points here the GM will give you some chances to use it and it doesn't end up being 30 build points of flavor.
Be sure to get an extra initiative pass or two, via wired reflexes (the cheapest), synaptic booster (bioware, very expensive nuyen wise but low essence and hard to detect) or move by wire (the cadilac version of wired reflexes).
Note all of this assumes you are going with heavy augmentation and are not an adept.