Hello Steve & Roz and a warm welcome to our community.
Regarding the national healthcare system in Portugal (SNS), it is available to all legal residents (including expats) although there are charges for some of the services provided. Have a read through the following page on our site and there is also a link there to the SNS website.
Regarding private medical insurance, I think you will find the fees much less expensive than those in USA. There are plenty of companies you can contact for quotes but here is one to give you a starting point.
Finally, if you elect to see a medical specialist privately and without insurance, again the fees are nothing like those in USA. A consultation with a General Practitioner will cost Euros 40 to 50 depending on where you live and a consultation with a specialist at a private hospital can be as low as 14 Euros. It is quite easy to 'mix & match' between the national and private systems.
I hope this helps and good luck with your research.