Hi Windy, is there any reason why you want an ibc rather than a tank designed to go in a van?. Unless it is due to cost (I believe ibc's are pretty cheap), there are no benefits. As you say, you will have to put baffles in, but also, because of the applications they are designed for, ibc's tend to be quite tall, so will naturally be more unstable.
Another thing you might want to look into is your weight. Is it the 1000kg version you have? (assuming it is the current shape). The reason I bring it up is because manufacturers quoted payload's seem to be about as accurate as their quoted fuel consumption!. I have a transit connect 230, which has a 900kg payload, or so they say, so I would be fine with a 500l tank, right?. One of my customers is a farmer, a big farm, and they have a weighbridge on the way in. One day I went over it with a full tank, got a bit of a shock to find it was reading 2300kg, that is only 40 less than my gvw, and I don't carry a lot of other stuff in my van!. Of course, if you have the 1200kg version you will definitely be fine with 600l, but you have been warned!.