It might be difficult for you to get a credit card with a €6k limit but I guess it depends on your income.
I believe the downside to the 0% cards is that if you fail to pay them off within the stated time period - even if there's only a small amount outstanding on it - you'll be liable for all the interest that you would ordinarily have had to pay. And if that happens, the interest on 6k will be hefty.
If you're continuing to work during your course (it's not clear if it's full-time or not?), figure out whatever amount you can afford to repay each month and just get a variable loan over a few years. Over three years, the repayments on 6k would be less than €200 a month. Pay it off each month and clear it fully when your bonus comes in.
Another option might be a deferred repayment variable rate loan. Borrow the €6k, deferring the repayments - interest will go up during this time but it wouldn't be huge on a relatively small loan over a short period of time. When your bonus comes in, you can clear the whole thing straightaway.
It might depend on whether you are certain to get the bonus or not - you don't want to take the risk of not being able to pay off the credit card and getting whacked with a large amount of interest when you were trying to avoid paying any interest at all!
