For the plants that will be outdoors, you can use some time-release pelleted fertilizer to make your life easier. I mix alfalfa pellets into all my outdoor container mix for added slow-release nutrients and microbes, too. The "all natural" potting mixes you see at the stores are a large percentage coir now, too.
Your hostas will love the moisture-retention of the coir, but other annuals might want a mix that dries faster, and for them, I'd mix in some extra Perlite, too. You can get bags of straight Perlite at Lowe's or HD. It's expanded rock so strictly speaking not "natural" but it is perfectly inert and holds air pockets in the soil well, too.
