Sorry if we are confusing you!
If you are planning on having plants, a good substrate is necessary. Your substrate should have laterite or flourite mixed it with gravel or you can just use flourite by itself.
Some plants require a good substrate because they get their nutrients thru their roots. Other plants a good substrate may not be necessary as they get their nutrients thru the water column. This is where fertilization is required.
Depth of the substrate is sometimes depend on the plants also. Sword plants grow large root systems and it is recommend to have around 3"-4" of substrate. In any event, I would make at around 3" and sloping to the back to around 4".
Having a bare bottom tank, refer to as bb, is easier to keep clean. These tanks are generally used for breeding and growing out fry where cleaniless is essential as there is no place for bacteria to culture, uneaten food and feces to decay.