Do you have a regular doctor that you like? If so, call the office and ask what plans they like.
If you have a Medigap plan, Medicare pays their part, then the Medigap pays secondary. Medicap G is the best coverage. It covers everything except the Part B deductible, which is $198 in 2020. Find out what that costs. If you are ok with that, go for it. Otherwise, that gives you something to compare other options with. Medigap plans do not include drug coverage, so you have to get a separate Part D plan.
Medicare Advantage (MA) actually replaces your Medicare completely with an insurance company plan. There are some really good MA plans out there, so I would not just rule them out. And they offer substantially lower prices, because of things like narrow networks. Some of them offer extra benefits, though, like dental. Network is the biggest issue, but they can be good for people if their doctors are in network and out-of-state non-emergency coverage isn't needed. MA plans have out-of-pocket maximums, which Medicare does not.
You should be able to find an agent who will help you. Ask people who they use.
You can use the Medicare plan finder to look at your options.
https://www.medicare.gov/plan-compareIf you have health problems, and can afford it, get the Medigap G. Plug your drugs into the plan finder and pick the Part D plan that gives you the lowest costs.
The issue with chemotherapy is that these are usually Part B drugs, not Part D drugs. Part B is medical, Part D is pharmacy. With Medicare, you pay 20% on Part B drugs (and everything else in Part B) after deductible. You'll also pay 20% on Part B drugs with most Medicare Advantage plans. That 20% can be HUGE when it comes to chemo. But, as I mentioned, MA plans have OOP max. Medicare does not.
If you were in Michigan, I could give you more specifics. I don't know the Florida market. I'm happy to answer any questions I can.