Flowering plants rarely need two feet of soil, but that's a good depth for lots of stuff like tomatoes and potatoes.

The soil will probably compact down after a few weeks so maybe you should plan to add more after it settles.

With two feet of soil, the distance between your posts depends on the type of soil.

If it's loose, with lots of organic matter, it won't weigh as much as heavy clay.

Every four feet apart should be fine unless you have clay.

Steve,

Do you grow any fruits and veggies? I'm sorry, but I just have no interest in plants that don't save me money on my grocery bill! I would have several suggestions for growing those kinds of plants.
But even for the flowers, I have one on what to border the beds with. It's a simple fact that wood, being an organic substance, will rot eventually. And coating it with linseed oil might even speed up the process? It might feed the mircoorganisms that will eat the wood?
I don't have any rasied beds, but if I was going to build some, I would use those large, corrugated, plastic roofing panels sold at home supply stores. You could rip them to the desired widths with a circular saw. They would never rot, and if you used some other plastic material (maybe PVC pipes) to hold them up, they would last your lifetime, and probably your grandchildrens', too?

Tyrell

Have used lots of material for beds in the past.
My best by far are concrete blocks stacked 2 high.
Never had a problem with depth being 16" although 3 high is even better (24"). Have grown mostly veggies though.
I have also found 100' to be way long. I have to walk
1/2 mile to get to the other side of the bed. This only works if you're only working from one side.
Hope this helps

Tom