You dont need to buffer soil dude. And you are right about getting away with nutrient feeding the first 2-3 weeks in soil. But if you are using RO water, its good to add cal mag in your feeding. You can go full organic growing in soil but you can make things a bit simple with synthetic nutrients. If you wanna go full organic and not budget conscious, you can try Biobizz soilmix with their full range of organic nutrients. All available on Lazada. Synthetic nutrient is pretty friendly for beginners. Any nutrient suitable for growing tomatoes is fine. You can also get General hydroponics flora trio on Lazada. Quite pricey though.