I can't tell you how large a planting area you need but a 1200sf garden should be able to grow a LOT of produce, especially if you do succession plantings of fast crops likes greens and peas. I grew up with a much smaller dedicated vegetable garden than that and my folks canned and froze a lot of what we grew. There were 6 of us in my family. You might want to look into 'square foot gardening' for intensive planting in a smaller area. How much soil/compost you will need will depend entirely on how large the garden area is you decide upon. You will have to determine that before you know how much to add. If doing raised beds, it's pretty easy to calculate the volume each will contain (multiply the length x width x depth). If open beds, you'll need at least 6" of soil, more if you want to grow any root crops. Remember to keep the units the same - 6" = .5'. If you were going with the 30'x40' garden area again to a depth of 6", you'd need 600 cubic feet of soil or about 22.5 cubic yards. That's assuming you have no existing soil at your disposal. Here is a link that might be useful: square foot gardening

Several (many?) years back, I did a same bed experiment with a newspaper bottom over undisturbed soil on one end and double dug on the other end and about 6" of mostly compost over the whole bed. I saw no difference in production from one side to the other after the newspaper had decomposed enough for roots to punch through mid-season and fewer weeds on the end that was *not* double-dug. I have not double-dug a bed again since then and have no qualms advising everyone else to skip that time and labor intensive task in a raised bed setting, especially if they are preparing a bed off-season that will not be planted in for a few months.. My compromise to the expense when initially filling a bed for immediate use is to use mostly bulk mushroom compost augmented from my household pile for some variety. I line the bottom of the bed with newspaper or cardboard, then a thick layer of old leaves and/or leaf mold (which has properties similar to peat moss when it breaks down), topped with the (hot) mushroom compost and then right around initial plantings I use a purchased potting mix (I prefer Scott's moisture control) to shield them some from the active composting going on around them. The only vermiculite that gets in comes from old pot-plants and from rooting cuttings as the used medium gets dumped into the compost pile. The worms eventually mix up sand from beneath the beds for more consistent percolation properties. Over a heavy clay soil I might be more partial to using vermiculite, but over sand I don't need it and couldn't see much benefit over rock either.

I know nothing about raised beds. Never cared for the concept so can't help. The concept of micro climates has been interesting. For years I'd heard about micro climates. They never made any sense in most climates I've lived in...San Jose CA, Pensacola FL, Rochester NY, Germany and even in TX and AK. Yes, a few obvious differences like south facing for earlier starts in cold climates, but nothing worth having a special term. Here in Phoenix that changed. Moving a plant even a few inches in a garden made a surprising difference. I've found management of sun to be really important for plants that can't take 100% in order to extend the growing season for vegetables, many flowers, etc. I see it the same as frost management. Move a plant under a house eve or tree and no frost damage. Unfortunately I've found it so subtle that it really isn't possible to give any precise advice. What I've done is plant in pots around the yard and see how different plants react. When I find the best spot I design around that. Takes a few years, but that's what I did. And I have to do that every time I've moved in Phoenix (3 so far). I do have hunches now so it's easier. One issue I do suspect is soil in pots becoming too hot. I was transplanting a potted plant once, mid day in summer, and the soil in the pot was so hot I could barely touch it. How any plant survives those conditions is amazing. Now I'm shading pots. Another thing I suspect is many places say stuff like vegetable X requires N hours of sun per day. More sunlight is always better, AK in summer is crazy good. In Phoenix in late spring, summer, early fall, sunlight = heat = death/dormant, so we have to deal with that. But we have one advantage, lots of sunny days. So I think I can get away with slightly less hours of sun per day and still get some reasonable result. For me, I'm more of a try everything to learn my yard. I threw away my Sunset Western gardening book, stopped watching gardening TV shows, reduce reading gardening info online after moving to Phoenix. It wasn't relevant, it was misleading. Even talking to neighbor gardeners often steered me wrong. People can have wildly different concepts of what "you can grow X here". For example tomatoes. I've heard many people say they grow tomatoes here and when I see the plants I see an almost dead plant and no fruit. When they say "tomatoes grow here" they were talking about the plant, not the fruit. It gets confusing. I have seen pictures of healthy tomato plants posted, pictures of fruit and lots of people say growing tomatoes is no problem...I don't doubt them. They lucked into a great micro climate spot I assume and they then assume anyone can grow the same lovely crop in any location. I haven't found that to be true. And for sure you can't asked any questions of most people because they're so insecure on the subject they instantly get super defensive. Only once in a great while will you run into a "gardener" that can carry on a civil in depth discussion. My view on vegetables is shifting to give up on summer and focus on winter. It's easier I think to raise the temp here in winter with a greenhouse/cold frame than deal with summer's sun/heat/low humidity. Look at our temps, sun, humidity right now...looks a lot like most of the country's summer values. Even a crappy cold frame/greenhouse can push today's temps even into prime tomato growing range. That could extend the tomato growing season from say Oct to June, 8 months. Seem like a better plan for me than the 2-3 month season we have in the spring and again in the fall at least for some vegetables. That's only what I've done. Not saying it's how anyone else should or that it's right. I'm still learning.

On another trip to Home Depot, The HD guy was kind enough to "carefully" situate eight 60# bags of Quickcrete cement (480# total) throughout the car! I used the same Toyota method of calculating five adults (approximately five 100# Japanese people -- maybe realistically 1 adult and 4 kids equating about 500#?) I figured I was safe for the 1-mile trek home. I shoved the Quickcrete out the door and it's still sitting in a lump -- thank God for gravity! I can't imagine that much laminate flooring in a Prius but given CBS 60 Minutes' recent expose on Lumber Liquidators China supplier problem of mislabeling its laminate flooring as CARB2 compliant when it clearly wasn't, I'll believe anything. I'm ready to rip out my LL flooring in favor of real wood or tile... That shall be my next project after I finish my garden beds and my new money tree starts to bear fruit...