If you don't mind working on your hands and knees, nothing could be simpler to build. Bases often take longer to build than the oven proper. This oven does not have a base.
There is no actual correct baking height, but when building a wood-fired oven, make certain that the baking height is comfortable for you and the other people who will be using the oven.
Somewhere in the area of 36" to 48" is about right. We are talking about a measurement from where your feet rest on the ground to the floor of your oven. The floor is where you'll be baking your breads and pizzas. If you don't build the floor high enough, you'll always be bending over to see what's happening inside. If you build it too high, then short people or kids will have trouble reaching up to it.
Look at the photos. The low oven in Guatemala was built before the building was put in, and the area you stood on was raised so high that you had to bake while in a kneeling or sitting position. Not a good arrangement. The oven was even low for the kids.
The other oven has a baking floor at a good baking height.