Hello dear Client!
Here is my plan/idea and I must say it was fun to work with! Furniture I used was combination from: IKEA, Mondoconvenienza and Leroy Merlin, like you requested (and ubuy for 2 x TV (upper bedroom and living room)). I calculated budget: 3963.8 Euros. I chosed Rectangle hole for stairs, with stairs dimensions:

Height: 21cm;
Width: 70cm;
Depth: 22cm;
SIZE OF HOLE: 160cm x 80 cm. (With my calculation it will be minimum height 168cm for walking on stairs. If its not enough then it must be wider, 180cm x 80cm for example)

On the wall on upper part for stairs, I put a TV so it can be watched from the bed. Also EKET shelf is used under the stairs (can not be seen in floor plan) and doors are added on the upper floor for privacy and heat loss. WC on first floor is rearanged a little different than your current one, so you can choose for the future.

I hope you like my plan, for any further edition you can contact me via email after the results, have a nice day!