Is there a 72-hour lemon law in Iowa (and what other options might I have)?

No. There is no "72-hour" law in Iowa or any other state except CA, where you can purchase the right the return a used vehicle. Lemon laws protect the buyers of NEW cars from serious, recurring defects. They do not protect the buyers of used cars from mechanical problems. If the car has a warranty, have the dealer fix it, if you bought it as is, you pay to fix it. No matter what, you have no right to return the vehicle.