Upgrade ESXi 6.0 to 6.7?

Technically? Yes you can, because the R730 is certified for 6.7 and the license key is the same for all 6.x version.

Legally? It depends. If your SnS runs out years ago where 6.5 was the most recent version than youre not allowed to use vSphere ESXi 6.7 with that license key you bought long time ago. You can use that version which was the current one as your Subscription runs off.

Install and use vCenter?

Well, you can install the Eval version which ends after 60 days. Otherwise the answer is no because for a vCenter Instance a valid license key is needed. You need a vCenter Foundation or Standard one. You can not use the cheap vCenter Essentials!

Regards,

Joerg