Well, $1,800 was a steal in my opinion. I would have expected 2-3 times that, seriously. It is absolutely ok to ride them - they are made for it. Today is the first time I've ever heard there was a 1968 K0. Production of the K1 started late 1967 and was the more preferred 5 speed and larger, taller stance with conventional forks and gauges. I have a 1,300 mile K1 CL450 that I'm bringing back to life. The mechanicals are fantastic, but the paint and chrome suffers from age.
As an investment for any serious restoration cost - the K0 is the only wise choice at this point. Of course age and rarity help, so the K1's would be 2nd. As for a lower cost rider, find any K1, K2 and later buy it and enjoy the better enginnered 5 speed. If you want to invest and keep it long term, find another BB and enjoy the comments and questions every time it's pulled into a parking lot. That bike gets a lot of questions and on-lookers wanting to know what it is...
Gordon