Dungeon World works great for smaller groups (Though it probably wouldn't be as much fun with just one player.) for exactly the reasons you cite. Things tend to happen reactively - monsters don't, strictly speaking, get "turns", but rather, act when a PC rolls badly or doesn't act to stop them. Similarly, the game isn't nearly as 'party synergy dependent' as D&D - while each class has their own strengths, you don't need any of them in particular.
In terms of character creation advice, I'd mostly just let them pick what appeals to them. It's not an awful idea to have some sort of source of healing, but if they don't, things should still work out okay; Many classes have the option to start with some healing potions, which should be enough to see them through one adventure, anyway. There are definitely no "need to have" classes.
For an adventure, I recommend starting with some sort of adventurous situation, and then using the Principle "Ask Questions and Use the Answers" to get the players to help you build the situation into one that bears directly on their characters. For example
"You're standing on the winding path leading up to the foreboding fortress of the Wizard King; Gargoyles gaze darkly down at you from the battlements, and a horn sounds from within the walls. Hrothgar - what has the Wizard King done that you must claim vengeance for? Fiznulf, what secret errand do you have within the walls of the fortress?"
If the answers to the questions don't give you enough to work with, ask a couple more questions. "So you say he has abducted the daughter of the chief of your tribe? Why is that enough for -you- to go seek to rescue her?" or "You're looking for the Orb of Thastar? What does that do? Why do you need it?"
Play the game according to the agenda, principles and moves in the GM section - paying particular attention, in this case to the moves "Give an opportunity that fits a class’ abilities" and "Show a downside to their class, race, or equipment" - since these are particularly engaging in a small group.