you can collect rocks from anywhere, you must boil them in a bucket or something, just put them in the container and dump boiling water on top of them and let it sit for a couple hours,i do this two times to be safe.. the you get some vinegar,just regular vinegar and you pour a drop on each rock, if the vinegar bubbles on the surface of the rock then the rock is no good and you throw it away. if there is no bubbles then the rock is good,so u boil it quickly one more time to get the vinegar off and u put it into your tank..
for changing out your gravel,you need another container to put your plants and fish in, a 5 gal bucket works good or some big tuppaware container or anything that will hold about 3 - 5 gallons of water, you take your heater and heat the water,u put some water conditioner in then u add your plants and your fish to the container..
next u drain your tank,and scoop all the gravel out. keep the gravel in a bag coz you might want it in future tanks.. then u give alittle scrub with a toothbrush or something to the bottom of the tank and the sides,use some tank water for this. then add your new substrate.. theres thread about how to add new substrate so i wont go into that..
for the led it might be low light,so id say stick with marimo moss balls,some java moss,some java ferns,anubias,some hornworth,some anacharis.im sure theres a few more i cant think of now..
when you put your new substrate in,your new plants and fill it up with new water and add some conditoner you should be then able to add your livestock back in that same day.. make sure if u have a filter u float the filter and the filter cartridges in your container with your old tank water and fish while u redo your tank,if the filter dries out youll lose all ur good bacteria in it and start ur cycle all over again