Currently i am using Roots organic soil by aurora which is sold through Aqua serene... This is a really good mix.
There are others from fox farm such as ocean forest, light warrior, happy frog and such.
I use the Roots because it has Oregonism in it which is a blend of different mychorizzae. Mychorizzae are a beneficial bacterial that grow around the roots and root tips and actually break down the nutes and minerals in the soil making it more readily available for the roots to absorb. I would strongly suggest investing in some type of product with this in it.
If you choose to use a soil which does not have this... You can always order BMO (blue mountain organics) off of Ebay. They have great deals and are always sending me free samples and extras. They also have very reliable shipping which is always nice. They offer a variety of nutes such as Super Plant Tonic ( contain mychorizzae) Grow it Green(vegging nute) and Flower power. They also carry composted chicken manure, bat guano, and worm castings which you can use to add and amend your soil with.
Worm castings are a GREAT amendment for a light mix for young seedlings... BUT you dont want to add more than about 30% beacause it becomes to heavy and hard for roots to grow through.
Instead of using diffent soils for each growing cycle I LAYER mine in 3 different sections. Top layer: 60% root soil 20% EXTRA coco and 20% extra worm castings. Roots has both worm castings and coco but I like to make it nice and fluffy on top for those young roots. Middle layer: 80% roots 10% extra coco 10% extra worm and Two tablespoons of COMPOSTED chicken manure. Do NOT use fresh chicken manure as it will burn your plants. Bottom layer: Roots plus high phosphorus bat guano.
And yes you do need to flush. 1-2 weeks before depending on who you talk to.
Glad to help friend
Peace and Love