Those wood arches, beams, pillars, and trim pieces create the perfect places to divide up the model of the walls and ceilings. No one is going to notice a modeling seam if it's hidden by a architectural seam. As far as getting everything into place in engine, UE4 has a helpful grid snapping function you can use. Just build…
You're going to want to break that up into modular pieces rather than one whole piece. It'll make it easier to model and texture and give you more flexibility when putting it together in UE4