I mix sunn hemp and buckwheat because they complement each other. Sunn hemp is a legume. I bought my first sunn hemp a number of years ago from Hancock Seed because they had free shipping on 50 lb bags. They quickly realized they were losing money on it and now only offer free shipping on 25 lb bags and you are paying more per pound as well. Since then, I've purchased sunn hemp from the local coop. Keep in mind, we always pay shipping in some form or another. They don't carry it and have to order it, but they are constantly buying seed from suppliers and have trucks already gong from sit to site, so their shipping cost is less than if we buy from some internet supplier who is shipping one or two bags of seed from their location to our address. With the coop, they are shipping an entire truckload from some supplier to the coop and my bags are on the truck.
I don't know what you objective for the plot is but there are some things to consider. Some warm season annuals like beans, corn, milo, etc are planted in the spring. Some of them like beans provide good deer forage during the summer like beans and then pods for fall/winter. Others, like milo, provide only fall food. Corn provide food through the cold months. Other warm season annuals are focused on providing only summer food.
I'm far enough south that summer is a stress period for me and that is the purpose of mixing buckwheat and sunn hemp. They both provide summer food and are fast enough to compete well with my summer weeds. However, come fall, they provide nothing. I replant for fall. One year I tried adding milo. That did not work out for me. While the milo grew well, deer use it in the fall when the seed heads ripen and by then it is too late for me to do a fall plant, so I ended up mowing the milo before the deer could benefit.
So, depending on your objectives, these are things to consider.
I think buckwheat does well with heat. It germinates best with a soil temp of 80 degrees. Keep in mind it is only a 60 to 90 day crop as far as deer food goes. Moving to no-till/min-till can help conserve soil moisture in droughty areas.
Thakns,
Jack