I, personally, prefer compost as the source of nutrients because it's slower to release - you can burn your plant with miracle gro, but if you put in too much compost the thing that would happen first would be the plant drying from drowning (compost holds a lot of moisture).
The big problem is that a new gardener doesn't usually have enough compost on hand to do their entire garden.
It can be purchased, but even "organic" compost is a little iffy.
Someone once told me something akin to "The best gardeners are growers and tenders of the soil, plants and edibles are a side-effect of healthy soil."
Think of inorganic fertilizer as fast food for plants. It works for what it is designed to do - help provide the macronutrients that help plants grow fast and big.
Organic compost is more holistic, feeding the micro organisms in the soil, who in turn help facilitate the movement of nutrients to plant roots, help good bacteria and fungi multiple, provide increased air circulation and moisture holding qualities (assume this is valuable in your area), provide food sources for worms who break materials down even more finely, etc. Compost also has micronutrients that you likely want available in your plants so it becomes part of your harvested crop.
Inorganic fertilizers have a role, but I have never used them in my gardens.
Horse manure can be problematic depending upon the horse's diet. Many horses are fed large amounts of mineral supplements (i.e. zinc) that are never digested and end up in manure. These supplements can accumulate very quickly when applied as compost and start wreaking havoc on your soil.
I would reccomend looking into getting the manure tested. I personally know of two different farms that have had large problems due to bad horse manure.
If you would like to do a raised Lasagna Garden the layers are:
Bottom to top:
- Horse Manure
- Dead Leaves/ Hay/ Grass (Whatever you find)