It is a cute tank, and it makes me happy seeing someone research and ask questions :) Shows you are a responsible pet owner.
And I wouldn't worry about the heater too much then.. these fish are pretty hardy.. heck, for many decades millions of them have lived years in just small bowls with little water changes. So living in a tropical climate and having no heater isn't going to hurt!
As for siphon.. if you can find a small tube of sorts that would work too..
With live plants you don't want to be doing 100% water changes, so a siphon is a must. Any kind of tubing would work.. 2-3 times a month is all that is needed, as the plants will use some of the waste as fertilizer. But there will still be waste in there, so it's good to clean it out every so often.
Just make sure the tube is smaller though - don't want to remove the plants when siphoning it out.. so want to be able to go around them with a bit of ease.
50% water change once a week is all that is needed with live plants in the mix.
If you use a tube rather than an aquarium siphon, make sure the water you siphon out goes into a bucket, as I'm sure you'll pick up some of the gravel with it and you don't want that going into a sink - a tube can be tricky to use.. so I would recommend a gravel siphon first if you can get it.