You can totally do dirt under sand or under eco-complete if that's what you were talking about, though you don't really have to spend money on eco complete if you have dirt, it's one of the cheapest ways to get a nutrient rich substrate. In the states, I recommend Miracle Gro Organic Potting Mix, so if you look that up on Amazon, you can see the kind of soil you're looking for. No fertilizer added, no perlite (the little white things in some potting mixes, they float when you add them to a tank).
You can't do an under gravel filter with dirt, however, it will just get clogged.
Backstory: I started with dirted tanks because I love the idea of a complete eco-system in miniature. So yes, you can start with them and be successful, I now have six and have helped 2 friends start successful dirted tanks as well.
But a good dirted tank is an eco-system, so you need snails. I actually like ramshorns. I have nerites as well, but nerites don't have a neutral shell like ramshorns do. Ramshorns (and plain old pond snails too) can climb your plants and eat algae off of them. Nerites stay mostly on the glass and wood. Also, if you are going to go with dirt under sand, you really need malaysian trumpet snails as well. They live in the substrate and help keep the sand from compacting too much (rinsing it VERY thoroughly also helps with this I've discovered as you wash away all the super fine bits that not only cloud the tank, but make it easier for the sand to compact).
As far as why to plant heavily: I have a tank set up in a window with no algae because of dense planting. The plants outcompete the algae because they are better at photosynthesis, but if you don't start with enough and algae gets a foothold, it can be time-consuming to get rid of. Although, my favorite solution is just to add more plants. I got a case of staghorn and black beard algae in another tank and I spot treated with hydrogen peroxide (it's a 3% concentration in the States and I dosed no more than 1 ml per gallon a day, from a syringe directly onto spots of algae). I added ramshorns and Amano shrimp to that tank because both will eat the dying hair algae types. And I added more plants, and surface plants. Within a month, I was down to no visible algae (I'm sure there's a little somewhere!) and I never have to clean the glass. If you don't overfeed, the ramshorn population will stay under control.
Diana Walstad who is one of the leading voices on dirted tanks recommends a tank that is at least 70% planted when viewed from above to start with. Floating water sprite, frog bit, duckweed, or the like helps because plants in contact with the air are much more efficient at using up nutrients in the tank because they can exchange carbon dioxide directly from the air instead of the water where the concentration is likely to be lower.
Lastly, if you have lights on the tank, set them up with a timer and give them an off time in the middle of the day of at least 4 hours. You can try something like 5 hours on 4 hours off 5 hours on. Or on most of my tanks its 4 on 5 off, 4 on because all of them get some degree of at least reflected sunlight (lots of windows!). Plants need a 4 hour window of lights on to photosynthesize, but turning them off disrupts the algae's ability to photosynthesize.
Hope that helps! Good luck!!