BigHuf;
I am a bit over 300lbs also. My commuter is a 1985 Schwinn World Tourist that I purchased new. Last summer, it went through a series of relatively low cost upgrades, including 36/40 Suze-Araya wheels with sealed bearings. The bike was made by Giant in Tiawan, and the frame is low carbon steel. I ride 12 miles each way, every weekday on it. No issues.
I am presently building up another bike, I purchased the frame last fall, it is a SR Sierra Sport made of Tange straight guage Cro-Moly. It is also mid- '80's vintage. I got the frame off eBay.
If you want to build up a bike, I recommend looking for a Cro-Moly frame; it does not need to be new. There are some great deals of low and mid range components that work well. They are not a light as the high range stuff, but work well, and often will last longer.