There are still four whole months before this match, so now it is quite difficult to say who or which team will be in what form and to make a prediction. I do not think that Portugal is much stronger than Turkey to speak of a clear advantage, at least I hope that the match at home will give an additional advantage and Portugal will win.
Turkey having some good players with good team but still not able to handle top ranked European teams those are having massive advantage in quality with some very skilled players I am sure Portugal will never have any issue against Turkey, and they can beat them easily but still issue is Italy where they are going to have some very tough competition and these both teams are level in quality and skill but in my view Italy is some better as they won recently European Championship, but we can't predict about any match which is going to play after nearly 3 months with many things can happen like injuries and some other hopefully this could be entertaining match for soccer fans around the globe.It all depends on the form in which the teams will be in a particular match - this is not a tournament where the result is more or less predictable and fair, this is just one match where each team has a chance.
Even if Portugal is really much stronger than Turkey and has, say, a 75% chance of winning, then given the match with Italy (I estimate it much stronger than Portugal as about 75 to 25 in Italy's favor), Portugal's chances of getting a ticket to the World Cup are about 0.75x0. 25 = ~ 19%
