When trying to disable a vehicle, is there a particularly good place to hit them to cause extra damage (such as the engine block or cab)? Or do hits in all locations cause the same damage?
There is no location-specific damage to any pointstreak vehicle that I am aware of. Fire on it however you please, they have a fixed amount of health and each of your bullets depletes some quantity of that, depending on the weapon and how far you are away (in some cases)
In general, the only pointstreaks that are feasible to shoot down (without a vehicle lock-on launcher) are:
(it's not impossible to shoot down other killstreaks, but in terms of the flow of the game and the amount of fire needed, you won't be in situations often where it makes sense)
Each of those tends to require a good number of bullets from an AR or a LMG in order to kill. The AH-6 tends to take 2-3 clips from an AR in the general case. The UAV and recon drone have far less health but can be tricky to hit.
The problem with this is that in most situations you are going to be sticking out like a sore thumb on everyone's radars while you're firing into the sky. Additionally, you're vulnerable, since you're not watching what's going on around you.
I strongly advise taking a launcher secondary with Blind Eye on one of your classes, so that if you find yourself in a situation where the enemy is calling in multiple air support vehicles, you can defeat it more easily. Remember that in MW3, shooting down air support vehicles counts towards your pointstreaks, so it is as valuable as a single kill plus it benefits your team by negating a killstreak reward.
The AC-130 is one of the more tricky vehicles to shoot down regardless of your loadout, since it tends to require more launcher rounds to kill than you tend to spawn with. (It and a few of the other high order air point streaks have "flares" that negate missiles shot at it when it first spawns, and after a cooldown) Coordination between a couple of people is generally key.
Also note that the EMP will destroy almost all killstreak rewards, so having it as your top tier in the support package will make your life in the late game quite a bit easier.
Most "ground" devices (sentry gun, remote sentry, for example) can be temporarily disabled with stun or flash grenades, or permanently disabled with EMP grenades. The disable duration is generally enough that you can get around the gun in most situations, and take it out with your knife.
The assault drone is a weird one. Upon further review, I'm not sure that EMP grenades disable it (or if they do, the radius of effect is small and hard to aim since it moves). A knife hit or single semtext won't take it out, and it requires multiple clips from most ARs and SMGs to disable. Luckily, it has a decent reload delay and is aimed manually, so you can use those attributes to your advantage.