That doesn't happen any more. Instead, each battlerank for your first 15 battleranks gives you 100 certs. It's a much better system since it'll end up giving you more certs overall and you can spend them on what you want. It's probably recommend to purchase things like better repair tools, better suit slots (e.g. upgrading your advanced shield capacitor, nanoweave armor, what have you), better abilities (upgrading jumpjets for light assault is an absolute necessity if you plan on playing the class), medkits or C4 (medkits are the more reliable, more often used options but C4 is fucking C4, you get to blow up MAXes and tanks with it), and upgrading your starting gun before getting new guns, with the biggest exception being the infiltrator for TR and VS, who starts off with the shittiest rifle in the game for their default weapon instead of something actually servicable.
All your default guns are at least passable unless you're an infiltrator that isn't NC. If you desperately, desperately want to upgrade, you would probably want the following
Light assault/engineer: The jaguar. It's a lot similar to the TRAC-5, but its recoil is arguably better, I'm pretty sure it has access to soft point ammo where TRAC-5 doesn't, and above all else it has .75 ADS speed (you move 75% of your walking speed when aiming down the sights as opposed to 50%).
Combat Medic: The default T1 Cycler is best in class for overall purposes. All the other guns are more or less great in their respective, specialized areas (except for the T1S variant and the T1A directive reward variant of the default Cycler) but there's no beating the Cycler in being good in most any situation you can throw at it.
Heavy Assault: If you absolutely have to, you can drop the CARV for the MSW-R. MSW-R has better recoil statistics and also access to an advanced laser sight, if you're the kind of person who likes hipfiring their LMG for some reason. I wouldn't recommend it too much, though, as it does trade off having a 100 round mag for only a 50 round one, and it still does cost certs.
Infiltrator: The one exception where you desperately want a new gun. Get the low-zoom bolt action sniper rifle, the TSAR-42 if you want a more midrange role, the Armistice if you want to go close quarters bullethosing with the other guys, or the RAMS .50 if you want to stay as far away from the action as possible.
If I started up a new character and wanted to play Heavy Assault first, I would do something like this:
1: Get a medkit.
2: Get a forward grip for my CARV because that thing shakes like a crack addict.
3: Get another medkit, probably. 2 is good enough for most players unless you start getting addicted to chugging those things.
4: Start working on putting ranks in my advanced shield capacitor, also use resist shield instead of the default overshield. Resist shield is strong from the get-go, so I don't need to put any ranks in it at first. Advanced shield capacitor, however, is incredibly useful and cheap and should be maxed straight away.
All of this is doable in your first 10 ranks or less and is going to do you a hell of a lot more than trading in your CARV for an MSW-R. From there, I'd probably go for getting the grounder (G2A rocket launcher, ludicrously useful for anti-air work, which is always welcome) and then maybe go get the commissioner (hands down the best sidearm in the game, and can be used for every single class).
