At level 18, IP is a huge issue.
Get Annie. Once you start towards ranked, you'll need to learn Support. However, Annie is still very good mid and out-ranges a lot of annoying pub stompers like Heim's turrets. She has incredibly attack range, a beautiful passive and great burst. Mid or Support, she's incredibly easy to learn, easy to buy and gives you one of the best entrances into support should you ever try it.
-Note: If you'd prefer a top flex as well as a pick that is more interested in poking as opposed to all-inning, Morgana is a mid/top/support mage, also easy to learn and incredibly influential from any position. She doesn't carry as hard from any position, but her Q is instant death in a team fight, and her ult is insane once you have Zhonyas (in fact, she's one of the best ways to learn how to use Zhonyas right where Annie with the
is one of the best ways to learn how to use flash right).. She has good bulk for a mage, too. However, she costs 1350 as opposed to 450. Probably get one, not both, this early.
I would get one tank, at least, as the meta is really, really good for them. In 1350, there's Mundo, Malphite, and Cho-Gath. The most versatile is probably Cho-Gath, as he can mid, top and jungle. He also scales insanely well with the cinderhulk. However, Malphite has a huge impact ult and is so incredibly hard to kill... probably more so than any other tank besides a fed Mundo or Maokai. Really, due to his flex, I'd suggest testing Cho right now (he's free) and if you don't like him, go Malphite (who can top or jungle).
I mean, you really look like you enjoy assassins, but they just don't fit on every team... oddly enough, you seem to enjoy utility adcs when you have such a preference for assassins. Anyways, don't do nid ADC... I don't think it really works anymore, and especially not when you have such great picks like Sivir (Chase queen) and Tristana (tower taking queen) as options.