Hi Mirko
Welcome to Wetpixel!
It's always very tempting to use the camera you already have and then pursue the housing etc.
I think a few of us though would offer a word of caution. €1750 is an awful lot of money to spend on a housing (and lens) and bits for a camera as old as the D3100. The housing and camera will have almost no resale value if, when, you decide you might want to upgrade it.
In terms of technology and key underwater elements such as autofocus, the D3100 is really old. You may well be able to put together a much better system for less money by taking a look at the Wetpixel Classified s and see what more modern systems are on offer and the price. This will give you an idea whether it's worth putting €1750 into what is, sorry, an effectively obsolete system.
Bear in mind too, that a housing a lens (and its port) are only the start. If you are going to go down the DSLR route, you'll need a strobe (maybe two) and arms etc. These are long-term investments in your system which yo make have already planned.
As I say, I can certainly understand you wanting to use the camera you have especially if its in good condition. But I'd be very wary of spending that sort of money without seeing what else you might be able to buy first.
As I'm sure you will see my moderator colleague, Chris, has written many times, when you start on a DSLR system you are looking at a total package of expensive gear. You need to see it in terms of a total package from the outset - not just buying a housing.