I would suggest you to buy it at the station. I traveled across Germany with a EuRail pass and within Germany it doesn't mandate any reservations. Keeping that in mind, I was always able to get a seat.
My parents traveled there last summer too and they although they had a EuRail pass, one of the members who they were traveling with didn't. They were able to purchase a ticket for him on the spot without any significant additional costs.
Why would I suggest that you buy it at the station? Simply because it's more convenient, I'd rather buy a ticket at the station for a train I'm sure I'll make, than make sure that I can make that one specific train which I have a ticket for.
In terms of prices, I'm fairly certain that buying in advance would get you a cheaper price, but it won't differ by much and it's certainly not worth the hassle.
I also tried looking up your specific train as per the information provided in the comments, but I don't seem to see an option using the IC either.