Foreign ministry for Westerwelle though is a given.
But as "Der Spiegel" itself says it is unlikely that the FDP will get both "economic" ministries.

In the past they usually got the economics ministry.
Plus justice and 1-2 less important ones.
Of course back then they never got 14% or so. So they might insist on more this time.

Finance would be "free" now since Steinbrück is SPD. But normally finance is seen as more important than economics so the larger party would be reluctant to give it up.

On the other hand a FDP finance minister might help Merkel? Depending on who gets it of course.
Even a FDP finance minister might be more interested in getting closer to a balanced budget = less debt. And so might be reluctant to agree to large tax cuts.