Unfortunately I've been cursed with the job of taking out vehicles. On every server I play on I seem to be the only engineer (or the only one concerned with tanks and choppers), so I worry about them while everyone else worries about the objective.
Going back to the leveling, it's a pain, but at the same time you'll have everything unlocked by level 20ish, so it's not a huge issue. If you don't have something unlocked by then, chances are you don't play that class enough for it to matter. Out of personal preference, I like the RPG more than the CG, but I like the AT4 more than either of the others, simply because it's easier to take out choppers (without a tracer). With guns, I like the 9A-91, which is the starting SMG you get as engineer, and you can do fine without any perks, because the irons aren't too bad on any of the guns.
If you want some easy points, whenever you see an objective (in any game mode from either side), point at the marker and press Q. It'll get little arrows around it and when you take the objective or do something with it, you get extra points. Also, throw motion mines as sniper. They may be pretty worthless in hardcore, but they give you a ton of assist points when someone dies near there.
Get yourself a nice mouse that has different DPI settings so you can switch easily. I have a Lachesis that goes up to 4000, but I rarely go up that high. It's nice to have lower settings to snipe with or if your hand is twitchy that day, but it's also nice to have higher settings when you're in the zone.