EU Citizen (Portugal) trying to move to Spain.

Hello MrBondurant,
As a Portuguese citizen  you enter Spain or anyother member country with your ID (cartao cidadao or passport), without any problems, as you are coming from a country outside EU, i rally do not know but should be the same as you are the holder of a EU passsport.  In Spain you apply for a NIE number  through the Extrangeria Department of the Guarda Civil, that becames your IDentity number in Spain for work, etc..... its a  A4 green page (called the hoga verde), must carried all the time with your pass or ID (portuguese), you must prove that you have funds to live also. As your partner isnt a EU citizen, if you are legally the best is to have registration with the portuguses consulate, where she can apply also for the same NIE, otherwise its a different process.
As for the benefits yes they exist, but they do not apply totally, I know by experience forget help, etc.... in case your life gets turned upside down, and I have worked here as a Autonomo (self employed) do to family illness I had to stop and when we needed there is no help in anyway, even if the EU laws are there. Spain at the moment isnt great, there is unemployed quite high, even if they say the opposite. Because, they take in account the people employed temp (3 months) and then kicked out.  Cost of living is up everyday, electricity very expensive.  Keep way from towns with less then 10000 people, also if you thinking of Extremadura region (keep to main cities), Some comunities (provinces  are not the same), The last paragraph (cost of living, etc.) is based on my region, does not reflect the rest of spain.  Regards, best of luck