Go to your local immigration office, get list of requirements for the Non O based on marriage. The cost for the visa is 2k baht. Why would you pay more?
You will not find any insurer that will cover pre existing conditions. Good health insurance is not cheap. Expect to pay £2000 - £5000 per year depending on age.
Immigration is aware that agents are providing one stop shop services in Laos and Cambodia offering to provide full documentation and financial resources to obtain DTVs for between 100k - 150k. I have read reports in these forums of enhanced scrutiny for holders of these visas.