I think the weapons are pretty much a matter of taste. Hell, the starting weapons, with a few exceptions (the Vanu starting sniper rifle, for instance) are pretty good in themselves. Use the Preview feature to try out new guns every once in a while; there’s a timer on it but usually once a day or so, maybe? Generally, the guns seem to differ in feel and subjective quality more than stats, as the stats the game gives you are pretty worthless.
For Engineer or Medic, I’d think you’d want to decide how you want to play, focusing on support or combat. There’s a lot of overlap between Engineer and Light Assault, too, so if you go down that route you can cross-pollinate your weapon/cert buys.
I usually spend my certs first on the low-hanging fruit of armor/shield stuff and the basic class abilities (repair gun, medic gun, jump jets, etc.). After that, you NEED a good scope/sight for your primary weapon; for Vanu I like the 3.4x one with the chevron marker, but YMMV. Sadly, you can’t try before you buy, but they’re only 30 certs and they’re all useful I think.
Beyond that, it’s really a matter of what roles you want to play. If you want close-range ability, stuff like laser pointer sights are cool. Extended magazines are always useful (less so for Engies I guess), and there are usually options if you want to try medium-range accuracy enhancements.
As to weapons, I like to have a close-range option, a long-range option, and a general purpose option–which can be the same weapon with different attachments, or different weapons entirely. My Light Assault uses a Pulsar C, a Solstice SF, a Nova shottie, or a Pandora shottie, depending on what I want to do, but really, I could get buy with just the Nova and the Pulsar C probably.
