I'm slightly late to the party, but not to worry.
You'll find that crawling and extracting data, regardless of how selective you are with the fields you choose, is a breach of most directories' terms of service and, worst case, will see you on the end of legal action; it is, in its simplest form, theft.
You'll also find that most IYPs seed their data with false data, which is used as a basis for identifying crawled data in breach of terms of service.
Where does that leave the individual who wants to create their own IYP? Well, one of two main options really. There's the start from scratch model, or purchasing data from an external source; the latter being potentially costly, but moving you closer to critical mass, at least in terms of numbers of businesses on your site, far sooner than the latter.
The updating issue you mention is something of a potential headache; the ideal is to have each listing on your directory contacted and their details verified at least annually; it's possible to do this via e-mail for at least some of your listings, but what about those that don't have an e-mail address? Can these be verified by telephone? Are they left as-is and the onus put on the end user to inform of any inaccuracies?
Purchasing data from an external source still presents questions in terms of updates; charges they make for updated data may vary, based upon how often you wish for the data to be refreshed and the level of information you'd like to receive.
Lots to think on! First thing I'd be mulling over would be what USP could your directory offer over those already in the marketplace? What features would users want or find useful? Are there any reasons why somebody isn't offering these already?