The only site I know of that sells sofas to be assembled would be IKEA and their quality is not the highest. Plus, to me, no sofa bed makes for comfortable sleep.

Maybe give consideration to a Murphy type bed that folds into a wall or cabinet. A king-size is made of two twin-size. The fold-away could be site built and twin box units and a king-size (or Queen) foam mattress probably will likely fit up the stairs.

There are movers who specialize in moving furniture into places where they ordinarily won't fit. In New York they literally have specialized companies that take furniture apart and then rebuilt.

The other alternative is to have it brought through the window. This isn't cheap but again is offered in New York City where typically it would otherwise be impossible to bring large items into brownstone type conversions or other walk ups. It has also been used in elevator buildings.

Presumably if you have an architect you can afford to pay for services.