The elite specs (which are limited to expansion) are a bit stronger. That's why you'd want to play them, since you want to be at your best in PvP. Having said that, F2P builds are decent enough. For instance, GW2 playerbase is shit enough so that as a F2P MM necro, you should be able to reach gold.
Well if you already know you will buy an expansion then I would recommend doing it now. While all the content will be locked until you reach L80, by upgrading your account now you will unlock the TP fully, start receiving daily login rewards, and your raptor will be back!
I made a F2P account, ran a F2P Thief Condi build and all I did was run from side node to side node. In about a day or two of playing constantly, I placed into Gold 2 and just hit Plat this morning. Every single game, I didn't teamfight. I literally just put on a YouTube video, ran to one side node and waited and ran to the other.. 258 375 250 183 370 268 199 59 365