Your character history won't make a lot of difference. It won't change how much skills cost, or which skills you can learn. Regular characters will start with 31 AP (choose freelancer if you want to spend the points yourself). Recruits will start with 0 AP, but they can earn special perks to narrow the gap (just choose the recruit dialogue option every time it comes up during the first mission). Veterans will start with 120 AP (every skill at rank 3), but you'll only be able to use them once you've completed the game as a recruit.
You'll probably reach somewhere around level 20 by the end of the campaign.
After completing the Saudi Arabia portion of the campaign, you'll get to specialize your character. This means you'll get to tag three skills, which will allow you to advance those skills to rank 15 rather than rank 10. At the same time, you'll be allowed to reallocate your AP.
Your first time through the game, we'd recommend that you use pistols and automatic rifles, and tag Stealth, Assault Rifles, and Technical Aptitude.
Most bosses can be killed with a combination of Chain Shot (Pistols 3, 7, 10, and 15) and Brilliance (Technical Aptitude 8 and 15).
Other useful skills include Awareness (Stealth 2 and 5), which will put an icon over the head of every enemy, making them easier to spot, and Breaking and Entering (Sabotage 2 and 14), which will make the espionage mini-games easier to win.
Even if you're thorough and thrifty, you probably won't be able to afford everything that you find for sale in the clearinghouse. We'd recommend first buying intel, then guns and armor (you'll almost never find them in missions), and finally upgrades, ammunition and gadgets (you'll find them all the time).
For weapons, stability measures how much your targeting reticule widens when you move your character. Recoil measures how much the reticule widens after each shot. Accuracy measures how wide the reticule starts out. All three are important, but if you're taking most of your shots while hidden from your enemies (where you can take the time to line up your shots), then you'll want to focus on damage instead.
At various times during the campaign, you'll receive emails containing incriminating evidence about Halbech. If you forward these messages to Scarlet, then you'll receive a point of reputation with her, plus $1000. If you sell them on the black market, then you'll receive $5000. And if you sell them to Halbech, then you'll receive $15,000.
If cameras spot you, then they'll sound an alarm. But if they spot a dead / disabled guard, then they won't do anything.