All that is dictated by starting Origin is your outfit and weapon you are given in the tutorial. That is it.
You can use any weapon on any class, you can respec your Perks and EGO Powers at any time by pressing Y / Triangle in the EGO sub menu, 2 tabs over from the Equip sub menu in the Character Menu. It's a tiered cost depending on your EGO level. At EGO level 350 in the PS3 Beta it cost me around 3500 Scips. At that point, I had about 70,000Scips so it didn't matter. It costs 404Scrips when below Ego level 50 and 800+ afterwards tiering up from there.
You have 5 loadouts that unlock as you gain EGO level, and there is no EGO Level Cap. You can eventually max your perks in the EGO sub menu, but you can only equip a certain amount of them, so character builds are more about what perks you equip that you've spent points on, rather than what character tree you follow, like say WoW. Eventually you can max every perk, and have every power, but you can only use 1 EGO Power per Loadout. For example. I have 3 Loadouts avaliable and I believe 5 Perk Slots open to me in my PS3 character, I have my Decoy as my main Sniping perk, with a bunch of perks that help me take less damage while crouched and still, and higher critical damage. I then have another loadout with an AR and Shotty, with Blur, and Melee damage boosts from my Ego perks, and I can switch them anytime.
Felt like posting this maybe it'll help somebody lol.