Defense Class: Ranged Support
This class is all about the range. The more range you have against your opponents, the better off you are. In MW2, this means sniping. Just be careful you're not trying to snipe on a map that doesn't lend itself to sniping.
Primary: The Barrett is a great gun, with more accuracy and a higher rate of fire than the Intervention. The Wazoo and the M21 are both even faster firing and more accurate, but take two shots to kill instead of one. Which gun you use is up to your personal preference. For attachments, be willing to swap. Thermal if your opponents aren't the type to use Cold-Blooded. Heartbeat Sensor is great to let you know if someone is sneaking up behind you; just listen for the beep.
Secondary: You may be tempted to use a shotgun, but if your opponent is in medium range, you'll die every time. Instead, a machine pistol is your best bet. The PP2000 has virtually no recoil and can ruin anyone in front of you. The G18, especially akimbo, can put bullets quickly into your opponent before they know what hit them.
Extras: Claymores are good, but be sure you know how to use them. Also, to anyone using Sitrep, they're a giant 'sniper here' flag. Semtex grenades can help if you know someone is coming that can out-gun you. Smoke grenades can cover your escape, as well.
Perks: Bling if you want to put multiple attachments on your sniper rifle. Otherwise, Sleight of Hand gives you a fast reload when you're low on bullets, and the faster aim down sights can come in handy. No matter which rifle you use, Stopping Power will help guarantee a one-shot kill. Definitely use it if you're using a silencer. Otherwise, Hardline can help you get your killstreaks sooner, or cold-blooded can keep you off the enemy radar. Commando can help you with a last-second save, and Commando Pro is great for a quick escape. Otherwise, Ninja can keep you from being spotted by enemies with heartbeat sensors. Steady Aim Pro lets you hold your breath for steadying your sniper rifle far longer than normal, so if you prefer to take a while to aim, use it. Martyrdom is always a good screw you style deathstreak.
Killstreaks: UAVs help you know when you're being snuck up on. Sentry Guns can help protect your back. Harriers and Chopper Gunners/AC-130s are good to shoot for, but unless you're set up in the middle of the action, you might not have enough players to kill. You could also throw a Care Package out into the open and use it as a decoy, to kill any enemies who go for it.