Regarding Medicare Supplements... the best time to sign up is right when you turn 65 (Guaranteed Issue). There are also times like Annual Enrollment when you can sign up or change. Other than those times, if you change, you're subject to underwriting, which means your pre-existing conditions will not be covered. Also, if you were in the hospital within the last 90 days, you're not able to sign up for a supplement. My in-laws have basically a Plan-F in Massachusetts (where I am not licensed), they pay a certain amount and do not pay copays/coinsurance for Part-A and Part-B costs. Basically, with those plans, you could go to the hospital a few times a year and spend 2+ weeks at a time, and not pay anything over premiums. Supplements are regulated by your state. Some states like Florida have it where your premiums will not go up very much, but premiums are dependent on age.
At least Medicare Advantage plans with Prescription Drugs (MAPD) do not have underwriting, you can sign up at 65 or during Annual Enrollment Period (AEP), or other special enrollment periods. But, they are regulated by Medicare (CMS), and premiums/copays depend on federal funding. I'm not sure about other MAPDs, but with Humana (one that I am appointed with), if you have a specialist you like, you can call customer service to find out which doctors refer patients to that specialist. That way, you can meet that doctor, and they can refer you to your specialist. I'm talking about HMO (Health Maintenance Organization). With a Preferred Provider Organization (PPO), you go to whomever you want, but you pay less if you go "in network" (insurer has negotiated lower copays with them).