1. Yes, 6 total.
2. If your wife currently makes enough for your household size, then she doesn't need a joint sponsor. She will provide a letter from her employer stating position and salary plus a few months worth of her most recent pay stubs. If someone has not filed previous taxes, or does not file because their income is not taxable, then they need to include a statement explaining why they were not legally required to file taxes. If you still use a joint sponsor, make sure they include proof of being a USC or LPR.
3. You can check the forum thread here (yes, it is very long): http://www.visajourney.com/forums/topic/231026-civil-surgeons-who-dont-rip-you-off/ and call around in your area to find one: https://egov.uscis.gov/crisgwi/go?action=offices.summary&OfficeLocator.office_type=CIV&OfficeLocator.statecode=WA
