I would HIGHLY recommend a Millenium 2000 filter for your new tank. I own six Milleniums and they're BY FAR the best HOB
filter I've ever used (and I've used them all). Aqua Clears are also great filters but I can't remember what "size" is rated for 30 gallons (although it should be listed on the box).
Whatever you do- DON'T take the cheap route when you purchase a heater: cheapies will not last very long and have a tendency to wipe out tanks full of fish when they do crash. You'll want to invest in a quality, submersible heater: I recommend Ebo Jager: I've got one that's over ten years old and it's still my most reliable and accurate heater. The Deluxe Visi-therms are good choices too. A rule of thumb is 3-5 watts per gallon of water, so a 100 or 125 watt heater would be a good choice for your new tank.
There's lots of options and methods for cycling your tank. Bio-Spira should be coming back out in a week or two and I highly recommend it- it's quick, simple, and works. Otherwise, you'll want to go with either the "fish" or "fishless" methods (there's lots of info on the web for both).
As far as ideal beginner fish, the types you've mentioned are all good choices, although you probably wouldn't want to do a "fish" cycle with the cories, as they're a bit sensitive to water conditions. The danios, most tetras, and the harlequins (my favorite small, schooling fish, by the way) would make an interesting and active community tank.
Good luck and let us know if you need any further info or advice!