My free bike becomes a commuter
Yip, as a guy who prides himself on doing everything as inexpensive as possible(I call myself the cheap bastard) I salute you! You beat me, I actually paid for my bike so you win in my book!!
Here are some ideas for you to go super low cost but still have an improved setup...
1. Do you have a patch kit? If not, get one. If those goatheads end up causing you to go flat, patch the tube, don't toss it.
2. Bike light. Mine is a 5 dollar flashlight and 2 hose clamps. Much cheaper than a bike light. Mine performs just as well as a 30 dollar bike light.
3. You mentioned goodwill...I look for bike jerseys there and get them for 5 bucks each. Any dri fit shirt will work too, you're in the desert so you know sweat I'm sure.
4. Keep doing your own maintenance!! Keep the chain clean, and lubricated. I use simple green to clean the chain, very cheap compared to the bike shop stuff. Any lubricant lubricates. Some are better than others, but even cheap motor oil is better than nothing. If your gears shift flawlessly, keep them that way. Once a month or so dab a little lubricant on all the cables, and other moving parts on the deurailers.