To answer the first question: you want a printer who says they can do white. It's not a matter of 5-color printing, it's more whether that printer does that kind of thing.
As you can tell from a look at the Pantone swatch book, there is no white ink in conventional printing. The translucency of the few whites that are available makes them unsuitable for most projects and you'll not find many printers willing to go to the hassle without charging you handsomely for their trouble. (Contamination from traces of other inks becomes a major issue with white, because any defect is so visible. That means the press guys have to do surgical cleaning of blanket, rollers, etc. before loading white.) On dark stock you'll almost always need more than one pass to build up enough ink density.
For pure offset, you can use one of the neutral Pantone metallics to get at least a pale gray look. On colored stock, especially uncoated matte stock with some tooth, there is no metallic sheen. The metallics aren't terrifically opaque, so a white underprinting or multiple passes are often needed. The standard advice applies: talk to the printer and see what they advise with their equipment and the card stock you're planning on.
For real opaque white, your best bets are thermography or white foil stamping. In either case put the text or graphic on a layer that's specified as "White" in the same way you would specify a spot varnish. Foil stamping houses generally prefer a separate file with just the foil portion, but whoever is doing the printing can advise you. Stamping involves a custom die, just like embossing, so be prepared for that to cost quite a bit extra for your first run of cards.
For your specific job, one "color" on dark stock, almost all printers want your text and graphics in black. You specify "white" in the order, and you can name the layer "white" just to remind them, but the objects should be black in the file. Same goes for foil and embossing.