Although much root activity occupies the top 6 inches of soil, many vegetables need 2 feet of soil or more.
The depth of your bed will be determined by what you want to grow.
- Shallow roots - 12-18" - Leafy greens, broccoli, cabbage, raddish
- Medium roots - 18-24" - beans, cantaloupe, cucumbers, summer squash and carrots.
- Deep roots - 24-36" - pumpkins, winter squash, watermelons, sweet potatoes, tomatoes, asparagus, and rhubarb
I recommend a double dug bed. Which means, digging down one shovel length, setting the soil aside, and then digging down one more shovel length. The length of a spade shovel is roughly between 8 and 12 inches, so this results in a 16-24" depth. Never have I actually dug down 3 feet for a garden. 2 feet is plenty.
It sounds like that is going to be a ton of work though. (I know, I've tilled up land just like you describe by hand. I ended up with a pile of bricks chest high and several feet in diameter.) So, with that in mind, you may want to dig down just one shovel length (12") and build a raised bed on top of that. It's a little more pricey to go that route, but you'll save yourself hours upon hours of labor.