Hello Christine!
From my experience (I have 1 rental in Lisbon, 1 in Barrreiro and 1 in Moita), for cash-flow and passive income the areas in the south of the river Tejo (Almada, Seixal, Barreiro, Montijo) you can get higher yields on rentals, because house prices are lower than in Lisbon, but rent prices are not very different. I would suggest to look at Barreiro, where you can find apartments below 100k (if you want to remodel you can find below 80k - 90k), and rent prices on T2 and T3 range from 500 - 700.
I would suggest also looking at Seixal (Corroios, Amora), its 15 min from Lisbon by train and also by car, and is a good market for rentals also. House prices are below 120k in some areas.
I also would suggest to look at Leiria and Santarem, as this markets are having increasing demand for rentals due to immigrants that are looking for places outside of lisbon, for more affordable living.
Wish you a successfull journey on your REI in Portugal!
Mauro Neves