A filter isn't absolutely necessary if you have a lot of fast growing plants, a low fish load, and keep up on water changes. You will want a powerhead or something to provide water circulation and surface agitation though. Personally I have never used a store bought filter with any of my planted tanks though I've always setup a DIY solution of some kind to provide the same benefits as a filter.
With a tank that size setting up a sump is the first thing that comes to mind.
Tough to make any recommendations on plants without knowing more about the setup. Lights, CO2, ferts, etc.