... well, my opinion is this: If that one job can earn U enough to buy that machine, and if You can see Yourself DEVELOPING - expanding your market to people that have those special, metallic or white needs - GO FOR IT!
Yeah, metalics print slow. Yeah, U need to shake cartridges. Yeah, it does have shorter life span, the color. But, U can easily work with those limitations, simply because your prints will look good, and will be for people that need their prints to look good.
And yeah, recently I found out it uses latest epson print heads, that produces 2 picoliters dots (for reference, Your SP's have 5 picoliters smallest dot size) - which should make those prints much smoother than Valuejets (3.5 pcl), at 1440 dpi. (not verified information, though)
One more thing: People, DEVELOPMENT! NEW THINGS, NEW TECHNOLOGIES! How do U think this all is developed, the sign industry? Someone made a printer one day and said: yeah, i am sellin' a printer that makes prints that last zilion years, have high resolution prints, photorealistic, and maintenance of machine is a breeze? It started slow and ugly, but "market" needed it. Is there strong market for metallic colored prints and whites? I'd say, HELL YEAH! (Unless U R living in miniature european country with 600k population, and even then, I'd buy it if I had the money to spare :{))