I bought my KO18 with Tap controls about 6 months ago. I love the oven and it can be a rocket if you want the temp to climb fast.
When I bought it, I was told the the difference between the KO and KF was two things. One, the walls in the KO are a little thicker. I went and measured mine and they are 3” thick, maybe someone with the KF can chime in and let use know what thier’s is.
And two, the elements are better to be able to get to the higher temperature.
This is a major one time purchase and my recommendation is to get the best you can afford. Even if you only use the KO for lower temp blade steels , the better elements will last longer. And you will have the better oven to do higher temps should you ever want to.
Erik