re lines, it does pay to get the best quality flyline you can afford, Rio is very good, but $$, Scientific Anglers are good, I am not a big fan of Cortland lines.
Re steelhead, while you can land a steelhead on a 5 wt, it is a bad idea. It is impossible to know if you will catch a native or a hatchery fish, and you would have to play them too long, endangering the fishes life.
Michael