I know you're trying to avoid metal, but since aesthetics are the issue, I thought I'd jump in and recommend Infinity. They make some fantastic models. For example, my Vindicare stand-in:
Also, if you don't mind using standard Cadians and doing some conversion work, sniper rifles are actually remarkably easy to convert. You can make long-las by splicing together two lasguns (cut the end off the wielder's weapon, cut a spare lasgun in half, glue it onto the end; lasgun is now 1.5 times longer), and the Heavy Weapons Team sprues provide a pretty nifty little scope that could pass for a real-life ACOG/Holographic.