I have cards to buy a road or settlement, can I buy settlement first and not place it on the board?
No, you may not.

As per the rulebook, under "Build" (on page 4) it says:


  Take the appropriate number of roads, settlements, and/or cities from your supply and place them on the game board.


An explicit part of the building process is putting the built items onto the board, so no, you can't buy it and place it later.