I'd check the Craigslist. An older Hardrock for around 100 should leave you extra for a tune up and whatever else you need. I would recommend prioritizing a rack/bag over fenders/chainguard for the time being. Other options (not sure of your height):
Panasonic Sport 1000 (1983?)
GT Palomar 16"
GT Singlespeed
Overhauled Schwinn 4130
?KONA Mountain Bike MTB Trail Save on Gas Go Green!
Raleigh 21 speed Road Bike

This is just a small sampling - the last two are more expensive, but still in your budget. The others would probably be fine and you would get your $$ worth from them. My philosophy is if the bike is of LBS quality and works and isn't rusted out, it is almost always worth 100 to someone.

Oh, and don't forget to budget for a couple of locks if you are leaving it out all day.