Your budget might be a factor as far as what pieces you end up getting. I have the Tunze Osmolator (199.99) which is at the top end of the price range when it comes to ATO's but it's been worth the investment for me. Tunze Osmolator Nano which runs for $99.99 but you lose some of the bells and whistles like the audible alarm and the lazer eye. If you are looking for something a little cheaper, there are various DIY posts on how to put one together if your handy like that.
For heaters, I have tried a couple different brands but settled on Eheim Jager for their reliability and reputation. Just get the wattage that applies to your tank size and make sure to calibrate it per the instructions. Videos also available on Youtube.
In regards to the skimmer; the consensus is that a skimmer is not a necessity on a tank that small and weekly water changes should be enough to keep a healthy tank. You can run one if you want. Tunze I believe has good nano skimmers that could work for your application.
Long post but I hope it helps. Research, research research, is the key.
