From your first sentance I can tell you that you don't really want a sump.
Plants and sumps don't go together well as sumps increase surface area and agitation making it much harder to keep CO2 in the tank. All that you put in will be driven out by the water falling to the sumpa nd being pumped back up.
Also, by doing planted you rule out the best feature of a sump: wet/dry filtration. The trickle tower is amazing in its ability to handle ammonia and nitrite, but massively gasses out any added CO2.
To build a sump you will need:
1) A system to get the water fromt he tank to the sump. The best method by far is to drill the tank abnd put a weir in with a durso standpipe. You can use overflows that work on a syphon, but they have more possibilities for failing, and I always plan that everything that can go wrong, will go wrong.
2) In the sump you will want some baffles (pieces of glass) to seperate different areas, so one for thw water to fall in and to put the heater in, then another area with filter media in, and finally an area for the return. You can have the water fall straight into the media chamber in a small setup such as yours.
3) A return pump to pump the water back into the display tank. MAke sure the pump is not too powerful for the drainage pipework.
So, if you still want it, that is what you need, but I would not bother if you want any decent amount of plant growth. Sumps are more for people who want to keep large fish than nice planted communities.
