I don't want to make any assumptions (this could be a fine art photo piece in a gallery for all I know) but I would be surprised if you really want or need to print at 150ppi for a piece this size.
As you say, your image would need to be about 1 billion pixels (1000 megapixels). Is your source imagery going to provide this? Are your audience going to notice the difference? You say "consisting of photographic elements" - only?
Also, there's no point in upgrading your PC and providing a GB-sized file if the printer's hardware can't handle it. I would make absolutely sure of this before going to these extremes.
If the main thrust of the question is "how do the pros do it?" then the answers to What resolution should a large format artwork for print be? should cover it.
