Hennie

I actually started off the auction, meaning that I was bidding on the domain and instead of accepting my offer the seller sent it to auction. I am not the winner of the acution, I stopped earlier.

The domain is reasonable, but not more than that as this is actually one of the very few words in Dutch that would really need IDN. We don't have IDN because the need is very little, actually so little that we don't have the keys for it on the keybard and most users would have no clue how to make them by enterting alt-codes.
In most cases it is pretty acceptable to enter the same word in ASCII, for example cafe instead of café. However, in the case of dieten.nl the pronouncation of dieten is very different from diëten. As the latter is not available as a domain name, the singular dieet.nl would be much, much better, as mentioned above.
Arguably, the domain Dieeten.nl which is officially misspelled but at least correctly pronounced would be even better than dieten.nl