substrate matters, more than you think. I highly recommend clay substrates such as ADA's aquasoil (if you can afford it) or Red Sea's florabase. Not only do they provide your plants with essential nutrients necessary for their growth, but they also act as a buffer keeping your pH in the ideal 6.5-6.8 zone which is beneficial for the well being of most aquatic plants. Along with adequate lighting, fertilization, and CO2, well, let's just say the results speak for themselves:
but to answer your questions:
1) yes. better to have than not to have. if you are going to forgo the filter, you should at least have some sort of water pump to circulate the water to keep it from being stagnant (though some fish like this)
2) define complete eco system... you can make the system as complex as you want it to be. benthic invertebrates, swimming ones, snails, hydras (HIGHLY AVOID), all the way up to fish. and yes, a tank of that size can do that.
3) hairgrass, lobela cardinalis, anubias nana, java fern, java moss, cryptocoryne parva, moneywort are all excellent beginner plants. if you want something slightly more challenging, try the Rotala species such as wallichii.
4) online sells a better variety than what can be found at petco/petsmart. try these two places: http://www.marinedepot.com/