Light weight will soon no longer be high on your list i bet.
You can pay thousands of dollars to lose a few pounds of bike weight. Then add fenders, racks, panniers, clothes, flat kits, etc and your back up and over what you were at. When your combined weight is 225 lbs, paying a grand to lose 5 lbs seems a tad crazy.
Your also in Florida, no hills, once your heavy pig of a bike is at rolling speed, a lighter bike isnt doing anything. Lighter helps mainly with acceleration and climbing.
What I'd suggest looking at changing is the rolling resistance/efficiency and not so much the weight. Have a suspnsion fork ? change it. Look at changing your wheels to somewhere between 25-35 mm if they are larger.
If your budget is 500, you wont get much for that new. Look around for used, or just upgrade those parts that will actually give you a good efficiency increase. Or increase your comfort.
In 6 weeks your gonna be semi in shape and at least accustomed to the 18 mile commute. Weight of your bike will no longer have quite so high an importance. Im mid 40's, I do 18 miles each way, and i have mountains to climb over to do it, both ways !
Oh and get off the damn sidewalks !