I agree completely that you want to go deeper than 6". To really take full advantage of growing outdoors, you should definitely loosen up and condition the soil to 2 feet or the depth of the water table, whichever is shallower.
You don't necessarily need exact ingredients to amend the soil. The main way to improve a mineral soil is to add organic matter. The best of the inexpensive forms is composted manure. If horse manure is what you have available, use it. If it's not composted, incorporate it NOW for planting in the spring. It's not a good idea to use the fresh stuff just before planting.
Agricultural lime (preferably dolomitic limestone) will likely be necessary to adjust pH upward, and it also supplies all the Ca and Mg you'll need, most likely. If you can send in a soil sample to a lab for pH and lime requirement, they'll tell you exactly what you need (tell them you're growing a vegetable garden, and you'll get good recommendations for weed).
Don't worry about oyster shells unless they're readily available. If they are, you can use them instead of lime, but I don't know how much Mg they have. Oyster shells can supply some micronutrients, but don't sweat it. You can supply them in other ways, if needed.
If your pH is right, your soil is amended with organic matter, and there is a population of earthworms in the area, you'll get plenty of worm activity, and free castings. If there aren't any worms in the area, dig some up elsewhere and bring them in. And not just one or two. Castings are great, but if you don't have access to a decent grow shop, it's worth employing a few of the local worms instead.
You should be able to get bone meal. But be warned that bone meal and dried blood can get dogs and coyotes in a digging mood.
I never use perlite outside, because I like to have some plausible deniability if the plants are found by LE. "I have no idea how that plant got there, Your Honor. It is a weed, after all." If that's not an issue, it's great for poorly aerated soils. If that kind of stealth is an issue, use sand. You'll need a lot more sand to replace a given amount of perlite.
The rule of thumb is that a plant's roots extend out about as far as its branches will when it's full grown. The plants I grow outside have always been sativa doms, so plenty of elbow room is a good idea. I have no idea how wide your strains will get.
Good luck. I love growing outside, but sometimes my nerves can't take it.
