Once you get AV status you could go for something like a cheaper M4 or AK. Those have the most accessories and upgrade available for them (V2 and V3 gearbox and any other gun that runs on those gearboxes like the MP5 and G36).
As for brand T7's written a post detailing the brands so you can decide for yourself:
Then there's always reviews and such for specific models.
In the $300-400 budget on the lower end you could get a CYMA AK47 for $300 and for up to $400 a used Classic Army or something.
For reference. My first M4 was $450 used, with a red dot included in the package. You don't have to spend that much on a first gun though.
PS: I'm a big fan of buying used if you can guarantee the seller is reputable and is very detailed in the assessment of the gun (ie. condition, scratches, use, any deficiencies or aesthetic problems).