Hi Ian,
I would recommend the Neutron then to start with and take it from there. You don't need a Eurorack to get started with a Neutron. Once and if you are happy with it, then you can consider a Eurorack.
Regarding the K-2, since it's a clone of an MS-20, it wouldn't be a clone any more if you would make it 1V/octave compatible, right? ;-) But other than that it is Eurorack compatible, just unscrew the screws, remove the front-plate module, remove the attached cables and you can put it in your Eurorack case.
To be precise it's the 961 module of Behringer, not the 061 (might be a typo I guess) :-)
I just recently acquire the Model D too, it's nice no doubt, however compared to the Neutron, I would recommend to go for the Neutron instead of the Model D.
Yes, it's terrible all these decisions ;-) Why we all can not just win in the lottery, so at least we have one worry less? ;-)
Kind regards, Garfield.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads