Pretty much any type of guppy is 'easy' to breed. I guess most of it depends on what size you want if you're going for feeders. I like the bigger type, so I usually buy fancy guppies so they'll get bigger. You can also just buy a bunch of feeders and see which ones end up staying alive, then from those you'll probably get a good bunch.
You don't need to buy show quality guppies, but chances are they may be more healthy. I've come accross a lot of unhealthy guppies.
What you should do is try to get them from different sources. This keeps the gene pool plentiful and you'll get better results. Try going to different cities to get guppies. Most of the time, all the stores in the town get their fish from the same source, but they're just taken care of differently in the store. For example, 4/5 pet stores in my city get their fish from the same place. In a city a mile away from my house, of the 4 stores I"ve been to, all of them get from the same place as well. So just ask them where they get their fish from.
Right now I've got some in a 20 long. I've got about 8 adult females and 1 male (this is a cycled tank I wouldn't recommend starting a tank with this stock). They're doing pretty good. Every once in a while use one of the adults as a feeder, and buy a new one.
As for choosing the guppies, I like to buy the females with the largest and darkest gravid spots. This will most likely mean even if she doesn't breed for me she'll likely drop her fry. For males you need to be careful. If you buy older guppes (most say 2+ years) they may not be as willing to breed as the younger ones. Go observe the guppies and look for the one that chases around the females the most, he's eager and willing to help you out.
Thats all I can think of off the top of my head, if you have more questions I'm more then willing to share my exp.