You can pvp and do instances once your max level. You can actually pvp starting from level 30 but you'll be at a noticeable disadvantage as you do not have all your skills(I'd still go ahead and play it though for the experience and daily quest, which can get you good exp and gold). It's either PVP or endgame instances.
It's kinda farm heavy if you want to get yourself to do the hardest instances available and get the best upgrades but for fairly decent gear such as defiance will be easy to get. Getting elite subscription will help you grind through things much easier and if you have cash to burn, you can also sell EMP(credits to buy costumes, accessories, mounts, etc) and sell that to other players to reach your goal faster. I'd say this is sort of p2w but can be debated on.
As for class tiers, I'd say lancers are top tier. They have pretty much everything in their kit and insta queue times(this is huge, as dps you usually have to wait 20-45 minutes for a damn instance whereas lancers and healers get in almost instantly).
I'd say berserkers and warriors are great dps for pve. As for pvp, I'm not too sure but slayers and lancers are great. Reapers are good for starting pvp though since they are pretty easy to use due to their massive aoe abilities and very slippery.