When we bought our small chest freezer 7 years ago, we found there was a tiny swing in efficiency between the best and worst. They use so much less electricity to start with that there just isn't much room to play with. We found the added cost of a more energy efficient one was not worth it.
One idea, our old Mpls grocery store (Cub) ran a sale once a year where you bought the chest freezer for 150$ and got 150$ worth of food coupons with it. You might only use half the coupons (I don't know what kind of junk it all was) but still, it was a pretty good deal. We actually found the best price for ours at HD, not knowing about the Cub deal. I didn't look used at the time, so I can't speak to that. My mom lives in Iowa and would likely to be willing to sell hers if anyone in the Ames area wants a used one.
We got a fairly small chest freezer, the second from the absolute smallest. It has been more than enough for garden produce, and I freeze a lot of stuff. But, big but, we don't do many meat, so that doesn't account for freezing meat. What we do do, is try to eat all the frozen garden stuff by this time of year, and turn the deep freeze off until July when I am freezing like crazy again. I am getting close to turning it off, a few more bags of corn and kale to go!