On K - there are now two "real" implementations, one by Kx Systems (i.e. the version under the hood of kdb+) and one by Shakti (new company from Arthur Whitney, original author of k). The Kx one is k4, Shakti is k9. Having said that, k9 is still quite a "work in progress", don't think it's considered production ready yet.
May also be worth nothing that k4 is essentially "undocumented" and Kx encourage to use q instead of writing any code in k directly. Most functionality can be pieced together using docs from other versions of k (I think k2 manual is floating around online) & the docs from q.