Eulychnia maybe? The spine structure reminds me of the genus.

Could it by any chance be a stenocereus?

I'm going to put my money towards acanthocalycium thionanthum.

This often happens to my Mamms. I find that spines just grow in eventually. I don't know why they do this, although it often happens if I fertilize frequently, so it may be because the plants grow faster than the spines can fill in? If your plants don't end up getting any, then definitely look for t...

It would be interesting (if you rub off all the flocking) to compare its growth to the "nudum" variety of myriostigma. In my limited experience with these plants there doesn't seem to be much of a difference if they're fully flecked or not, but maybe in the long run there could be a discer...

Yup, agreed!

In my opinion they look like offsets that have detached and corked at the base. From the picture, this is more apparent in the bottom left hand plant, as it has roots sprouting from side and not the bottom. While I don't think it's impossible, it is certainly doesn't resemble anything I've seen in m...

To me it looks like an Echinopsis oxygona.

You can't go wrong with any of the Echinopsis hybrids, but as for species I can recommend Echinopsis obrepanda, and oxygona. Lobivia pentlandii can have quite nicely bicoloured flowers, and its spines are good too. Lobivia ferox longspina is another one, and as an added bonus their flowers are scent...

I'd put my guess towards Echinocactus grusonii as well.

From my experience the bottom of most Ortegocactus plants go rusty brown at some stage. I don't know if there's anything you can do to prevent it, but at any rate, it's not dead IMO. As for the mites I have had good luck spraying with a dilute laundry detergent mixture (I couldn't see any in the pic...

Thanks for those links Dave! New Zealand has a few native/endemic species so I'm trying to start there. Most of them require a cool hibernation period so I'm looking for tropical species that would tolerate my greenhouse environment better.

Can anyone recommend a good supplier for Drosera seed? Preferably one that can send internationally and meet NZs import standards. I am getting into carnivorous plants and I'm finding it difficult to find any suppliers in New Zealand so I'm thinking I'll probably have to look further afield. If anyo...

Could the third one be Mammillaria candida?

Maybe Mammillaria microhelia?