I think you can interchange either the colt lower or upper with other brands, but you need special pin adapters, because colt decided to use a larger diameter pin. With the amout of supplyers out there and competitive pricing, Colt just isn't worth the hassle.

Two stage triggers.
I'm very fond of two stage triggers, especially on a semi-auto. They add a large margin of safety, and yet if tuned properly can have a break as crisp and light as any trigger. I second the recommendation of using a Rock River Lower equiped with the company's NM trigger. Should have a takeup of around 4 lbs, with a sharp second stage break of 0.5 lbs, and is more consistant than any otehr of the special AR triggers, including those costing twice as much.