Assuming you avoid Advantage you'll need something to cover what Medicare does not. One thing to pay attention to there is what happens if you get sick when you're out of the US. Some medigap plans cover international care, some don't.

That said, not all the Advantage plans are utterly terrible. For instance, the one by Tufts, which appears to be part of their group health plan, looks OK. If I didn't have enough to cover dental on top of medigap I'd consider it - but with some trepidation due to the issues you've mentioned in getting medigap if I decide to switch back.

Also, as someone who started Medicare at 65 but didn't stop working, it's NOT free - it just comes out of your Social Security payment when you decide to retire (costs about the same as medigap, which figures as the lower cost services get used more).

FWIW if you get hurt on a trip to New Zealand, they will pay for your emergency care, but I don't think they cover non-emergency. I just know this because I managed to hurt myself there. :-(