Sponge filters, I agree, they basically never wear out, though I suppose eventually they might start to fall apart. Treat them the same as filter pads in canisters or HOB filters. Rinse them so they don't clog (sponge does clog, as does any filter material intended to trap fine particulate matter); how often depends upon the tank and fish load. Every week during the partial water change works. If live plants are in the tank, they can be rinsed in tap water; if no plants, I would rinse them in tank water to avoid killing the bacteria.
Heaters, my advice is to buy the best. There is probably no equipment more important than the heater. If the filter stops, esp in planted tanks, you have time to repair/replace it before trouble (fish loss). If the heater malfunctions overnight, either overheating or no heating, all the fish could be dead by morning. I speak from experience. And the better heaters have a much better track record for reliability. The higher wattage heaters are the same, they work better. My 150w and 200w heaters have never failed in more than a decade; but I've lost 4-5 50w heaters over the same period when they just failed (some overheated, some didn't heat at all). There are some things where saving a few dollars is not wise, and the heater is one of these. For a 20g, I would get a 100w or 150w. I have read several times here that Stealth heaters are good. Eheim I would trust. My last heater purchase was a Fluval that has the LCD display of the temperature; I like this heater. I've had it for just over a year, as one of two heaters in the 115g, and no issues yet. I got it because of the temp digital display, more reliable than thermometers.
I would not use a powerhead on a sponge filter esp in small (20g) tanks. I have a sponge in my 10g, 20g and 33g tanks, hooked up to an air pump in the 10g and 20g, they work fine. The 33g is an enclosed Eheim sponge filter with its own motor; I believe they don't make these any longer, too bad, a super little filter.