A more sensible option would be for you to buy a condo in your name. You can have any legal framework to lease the land but if anything goes wrong with the relationship you can have serious problems.
Non O is your best option. No need for travel insurance through Thai approved Companies. Apply for Non O 90 days whilst in the UK. Travel insurance can be very expensive. Really depends on your age and health. I eventually took out annual travel insurance but cover is restricted to 55 days. Other UK banks can provide travel insurance. For Thailand Insurance providing heath cover I tried AXA Sawasdee. Alternatively try an IFA based in Thailand