Purchasing train tickets at the station

I will be flying from Paris to Dusseldorf, then taking a train from Dusseldorf to Cologne. The flight gets in at 9pm, but as I'm not sure how long it will take to get off the plane, get my luggage, find the train etc., I'm wondering if it is possible/recommended to purchase my train ticket when I get to the train station. I just want to avoid purchasing a ticket for a certain time (eg. 10pm) then missing it due to delays. Help please!
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