If you are thinking in terms of 3 to 5 years, then the word 'investment' doesn't really apply. The Silver coast has a mostly local customer base, and they aren't buying right now and are unlikely to get involved for the next few years. You are probably facing a slice of negative equity, which shouldn't bother you if you want the place for your own use -- its a nice part of Portugal despite colder winter weather than the Algarve.
Rental prospects? Not great, maybe a small bit in high season if you can find a good agent locally to manage it for you. Won't pay the mortgage if you have one, but might just pay the fixed costs.