Fancy guppies are super easy to breed. You want a minimum of 2 females to one male ratio. Otherwise the abundance of males can harass the fewer females.
Since they breed very fast, I would recommend under stocking your tank to start with. With a 20 gallon, I'd personally start with maybe 8. 5 females, 3 males. This doesn't give you a very large breeding pool, but it'll allow you some breathing room on water changes and filtration.
You could add as many as 12, but you'd have to get a beefier filter and do more frequent, bigger water changes. The maturing fry would put a strain on things, otherwise.
Since you're wanting to breed and the fry can be small, I'd recommend an air driven sponge filter set up. It's cheap, easy and won't suck in any fish. The
Tetra Whisper 40 is a good price and would be plenty of filtration, even if you wanted to stock up to 12. You could do a 20 if you under stocked. Two separate sponge filters would probably be better than one, in either case. I use these
double sponge filters in a 20 since they are a good, low height. The Tetra 40 comes with a T-junction for easy splitting.
As for temperature, guppies can be pretty forgiving and manage in temps between 75-82. Any ol' heater will do, but I use
Eheim. If the room will never be more than 5 degrees lower than the target temp in the tank, you can get by with a 50 watt. Otherwise, go for the 75.