Do "interaction" effects cause people to buy buildings in tandem in Puerto Rico?
I don't think you can just sit down with a strategy as precise as knowing your end buildings and stay competitive against a more flexible player.  The game does not have a lot of randomness but the distribution of plantations does play a huge role on the rest of the game.  

It's well balanced in the sense that if you are too out of control in one dimension then you are not giving the other dimensions enough attention and the other players should be able to exploit that.  Once you have a competitive advantage then by all means exploit it, but like most Euro games you should probably spend the first half building an engine and the second half running after VPs.

All that said, your engine will probably be built around selling, shipping, or building and each of them has big buildings which work best with them.  The wharf and/or harbor can be more powerful than one of those big buildings if used well.

In terms of defensive buying you have to decide if the person you are targeting is ahead of you and if you are in second by a wide margin.  The VP cost of the defensive purchase is the marginal difference in VPs between your ideal purchase and the one you are considering.