I wonder the same thing every time I see that picture of Rick's. Why aren't you doing water gardening extensively, Rick? You could grow outstanding water lotus, and all sorts of marginals in your low spots . . .
When we moved into this house, I had a landscape guy build me a 3' x 12' raised bed in the middle of the sunny portion of my back yard. My raised bed is 18in. high made of Trex so it doesn't rot and break down, thankfully. I had it built so that I could grow stuff, and weed without so much bending over. We have pure sand, too - not even any rocks here, so drainage is fast.
What I didn't count on was the invasive roots of four huge, HUGE oak trees around our house. Since that raised bed is good, rich soil that's been well amended and fertilized every year for 8 years now, and watered like crazy when things are growing, the oak tree roots have found it, and now, I need to hire someone with a strong back and a will to work to shovel all the dirt around and cut/sift out the oak roots before I can plant any more veggies in there. It, along with all my other border beds are completely inundated with oak roots. I think I'd have to make deep footers around the bed to prevent the oak roots from coming back too.
Moral of the story, sink a barrier of some sort around your raised beds if there are trees anywhere nearby. The tree roots will get in eventually, but not nearly so fast and not so many as I have, if you put down a barrier before you start.
Also, check with your County landfill, if they have a composting program. Most places do, and they make excellent compost. Here, the compost is free but you have to load it and carry it away yourself. In Salt Lake City, where we made a garden on pure clay in my daughter's back yard with 25 pickup loads of compost, they sell it for $25 per load, but they at least load it for you (if you have a truck) with a small bobcat. I would fill raised beds with at least half compost and half topsoil, but 100% compost might be even better, if the soil under the beds is good enough to mix with it.
When you fill your beds, be sure to over fill - mound them up really high, because the soil will settle a lot, and the organic materials like compost will break down substantially over the first summer as well. Where you planted your plants in 10in of soil to begin with, by the end of the summer, you'll find them struggling along in 5in. or less of good stuff.
Above is a pic of a raised bed we put in at the local elementary school. The Boy Scouts installed 6 of them as an Eagle Scout project, and they neatly filled the beds with compost right to the level of the top edge . . . This pic was taken in October, the beds were put in 6 weeks before and you can see how much the soil has subsided already, just from settling and watering. By the end of the school year we had to buy another dump truck load of compost to fill them up again (luckily we had a grant to pay for delivery, the Boy Scouts had gone on to better things by then) as the soil had subsided 6in. in some beds.
