Can someone assist me buying a PSU for cv.ocd?

It's supposed to be:

"9-12V DC via a 2.1mm barrel plug with negative centre pin"

I've found some of these, but what confuses is me that the wattage of different PSUs fitting into this range waries quite a bit.

This is the one I bought. It has the 150 mA minimum requirement. I've been using it for a couple months with no problems.

Where'd you read the 150mA minimum requirement? Is there a maximum?

That info used to be posted on the website, but now that they are selling a power supply with the CV.ocd, it's gone.
There's no max rating. The CV.ocd will only draw as much power as it needs. So just make sure your power supply is rated for at least 150 mA.

CV.OCD is a gadget which changes over MIDI signs to the Control Voltage (CV) and Trigger/Gate signals which are utilized to control customary simple synthesizers.

And doing this helpful piece of "plumbing", CV.OCD is intended to open up some inventive choices that go past what a straightforward MIDI-CV converter can typically do. In the event that you get a bit OCD about your CV, this could be the container for you! You can find here good quality PSU for cv.ocd.

CV.OCD has four assignable "persistent" simple CV yields and twelve assignable door/trigger yields.

It has capable usefulness for producing the clock, trigger and non-note doors from MIDI data. For controlling your simple hardware by means of MIDI you are presumably not going to get as much value for your money with whatever else.