Guppies don't actually need a heater as long as the room temp isn't lower than the mid 60's. A little sand for substrate would be helpful for the beneficial bacteria, also some fast-growing live plants like Najas or hornwort to maintain water quality.
I actually have white cloud fry growing out in a 5 gallon bucket that way, no sand in the bottom but there is some Najas and Salvinia in there, along with some ramshorn snails.