Door between basement and basement vestibule

Hello! We are building an EVH with a basement. The basement will remain unheated and the door between the anteroom and the basement is supposed to comply with climate class 3 according to the building specifications. Unlike the other doors in the house, this one is not part of the contract and is to be installed as self-performance. So far no problem, but I still asked the general contractor for the price, and we were offered a wooden door with the same beech decor as the rest for a hefty 1100 euros. I declined politely at first, as the price seemed very high to me and I don't necessarily need the "perfect look" matching all doors downstairs in the anteroom. Now my questions: Can I alternatively use an insulated steel door??? I was thinking of an "MZ-1 multipurpose door" from Hörmann, or something similar. The thermal insulation value is specified as K=1.7w/m, however, regarding the climate class, according to the description it does not seem to comply with class 3, is that a problem?? Regards Stefan