For an average 4x4 shower, with a sub 2K wall budget, people are using the cheap acrylic from box stores. For a 3-7K mid grade budget, people are using cultured marble, or budget 12x24 tile. For a 7-12K budget, people are using large format porcelain sheets, Corian, quartz, or natural stone. For luxury level budgets, they are using the same, but with exclusive luxury level slabs, or custom gemstone sheet goods like Prexury.

For low upkeep with a marble look, I would look at large-format porcelain slabs or large porcelain panels for the shower walls, rather than real marble. Real marble is beautiful but needs more maintenance and can etch or stain. For the floor, choose a porcelain tile with a slip-resistant finish and enough grout lines for traction, especially inside the shower. A quiet stone-look floor tile in a smaller size can pair nicely with larger marble-look walls without making the room feel busy.