I use 50w Aqueon Pro heaters in my 5g tanks, but you can also use 25w heaters. The 50w will be able to maintain heat up to about 15 degrees above room temp, while the 25w will only be able to maintain up to about 10 degrees. I also like Marineland Visitherm and Jager.
If you don't filter it you should do 100% water change a week, or actually I suggest every 5 days based on my own personal experience. I would also buy a small airstone, controlled with a couple airline control valves to turn it into a mist, under the heater to distribute the heat or you will have very uneven heating.
If you get a filter just place the heater next to the filter.
I don't use filters in my 5g, but I heard good things about the Marine S10 slim, and I think you can probably baffle an Aquaclear 20 enough (prefilter sponge, turned all the way down, and sponge outflow baffle) to be okay for a 5g. You could also look into a sponge filter, which provides excellent biological filtration but no mechanical.
You'll also need a siphon, reliable drops kit for ammonia, nitrite and nitrate, and lots of oversized soft silk plants, as well as at least one little cave of some kind.