My understanding is that US-ian expats can collect Social Security while outside the country (not sure if it can be deposited, as we're not doing that). Medicare has no meaning outside the US but we're eligible. Non- (and I assuming renounced) citizens can't get Social Security unless they live in the US, I think. And don't forget there's also a foreign tax credit as well as the Earned Income exclusion.

Non-citizens are not eligible for any public benefit. As regards medicare you're probably eligible the moment you enter the country. So technically you could head back if you 're having issues - ymmv.