No offense, but if you can truly budget
~€2 million for a 50' boat you can easily afford to pick up a phone
and offer to pay Chris White, or Melvin and Morelli, or Peter Johnstone, or any other cat builder
for a couple of days of their time working out a preliminary plan and budget. And you'll get way more accurate responses than anything you'll find here. Even better, if you really are in that € bracket, and they think you are, they'll probably do it for free to get a new, high-spending client.
If you can work with existing hull
designs (so no new molds or one-off builds are required) and only change the layouts, then you could probably do it for half your budget (and send the other half this way so I can get a copy of your boat
). If you want a complete custom one-off because none of the available hull
shapes suits you then you could still do it well within that budget but it will cost you significantly more than semi-custom.