>>I saw a Club 3D, it's about 50 EUR.

Careful, thats most likely one for the wrong direction.

I did some searching in the last days as i might try getting a 4k running at 30Hz (same bandwidth as 1440 at 60) and this is what I might try hunting down:

https://www.gefen.com/product/dual-link-dvi-mini-dp-converter-GTV-DVIDL-2-MDP

No idea wether it will work, but is at least the right direction.