1) Substrates generally last a number of years, especially if you keep the water column full of ferts. The top aquascapers like to replace it after each layout, but to be fair it'd probably last a lot longer. You're right in saying that it also depends on plant mass and species of plant, lilies for example are very heavy root feeders so if you have lots they'll run the substrate out quicker. Growth rate of the plants also makes a difference. I personally will probably replace it every few years, 2 or 3 depending.
2) PH controllers are ok but they are hugely expensive. If you have the money spare then there is nothing wrong with them per se, just if money is tight (which it normally is in this hobby!) then spend the money on other things, like a decent substrate or more light. They certainly aren't 100% essential.
3) Solenoids are useful but again not essential, they are fairly cheap thou (I got mine for 15quid). They are useful on bigger tanks as you can shut off the CO2 during the night which means you dont waste any. I have run tanks with CO2 24/7 no probs.
4) A substrate of around 2" is good, you can mix types, I've got black gravel and eco-complete mixed in my big tank. The obvious thing is that substrates with different colours dont necessarily look very good if mixed. For example sand and laterite.
Hope that helps.
Sam
PS - Welcome to the forum
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