I have used Tradestation for the Dax, they are considered to have one of the best data feeds. Their data feed for trading the Dax costs a bit less than IB and it is reliable.
IB is not free for real-time ICE quotes - like the DAX. It is true you can trade the DAX on IB without paying for the real-time data feed if you wanted to although your looking at delayed data.
Eurex is 10 euros. The IB software is free unlike tradestation. ICE is $110 a month and does not have the DAX.
Your right it's the Eurex - not ICE my mistake. With TS 10 round-trip trades or balance above $100k there is no platform fees. To trade just the DAX it looks like IB is cheaper at 13 Euros. TS is cheaper if your trading the whole spectrum on Eurex besides the Dax, commodities, currencies, & STOXX.
The official DAX index is from Deutsche Börse and is only calculated every second, using data from Xetra. Eurex has futures and options on the index.