Your composting plan sounds great. Many farmers here believe in and talk about healthy soil: soil that is rich not just in the right kinds of nutrients but also in organic matter and bacteria.
If there are horses or cows near you, I'd recommend filling the back of the Sienna with that too.
Cow manure is best, but horse manure will do just fine and is usually free. Enough natural fertilizer and there will be no need to buy soil amendments for a home garden.