"what sort of chance there is that the Pedido de Informação prévio would be successful" - no idea but a quick meeting with the camara, specifically with the architect responsible for the area, would give you an answer to that.

"Is it something I should build into any offer I make as a pre-requisite to the deal going through?" - if you want to play by the safe side, yes. This should be the rule for anyone buying a peace of land to build or with something built with permit.