First let me congratulate you for even considering to commute on that route, let alone doing the research (sounds & looks like some tough roads). Second, I hope you get the job, it sounds like it'd be a lot of fun for you.
If you're looking for a commuter bike, and staying around $500 max, I expect you will have plenty of options. An inexpensive road bike should be rather easy to find, and if used (but still in good condition), you'll get much more bike than you would new. See if you can find one that has the handlebars at the same height from the ground as the seat (it's a comfort thing, you WILL understand
), as many road bikes are setup as racers and have the handlebars up to 4 inches below the seat which 'leans' you forward more and puts more weight on your hands. I would advise against a flat bar bike as it reduces your aerodynamics, but if top speed isn't an issue they do allow more room on the handlebars for lights, etc.
Which brings me to.....I have a 12w halogen headlight (rechargeable lead-acid battery system, handlebar switch, mount needs some work, all that good stuff) that is not being used currently, and a couple of red flashing LED lights (one for the bike, one for the body). I'd be willing to sell them to you as a set for $15 plus shipping. I do need to test the battery since it was sitting unused for just over a year, so if it doesn't work well, then I'd give it to you for free (just pay shipping) as you'd have to replace the battery.
Good luck!
Vol
BTW, I'd recommend against the chain, as it will be rather heavy, and you will be less able to get away from the pissed off drivers than your sportbike riding friends are.