Just to add my two cents as well, from the list you've given, you'll need (my additions in blue):
Applicant:
- 2 Passport sized photographs
- Application form
- Appendix 2 financial requirement
- Letter of introduction
- Passport
- All previous passports
- stamped biometrics confirmation letter (once attended)
- shipping details
Sponsor:
- Copy of passport (certified)
- Letter of introduction/support
Financial Requirement:
- 6 months of current account statements
- 6 months payslips
- Letter from employer
- Official letter from his banking institution (only required if bank statements are printed from the internet)
- job contract (optional, but they may ask for it)
- P60 (optional, but they may ask for it)
-
Evidence of Accommodation:
- His tenancy agreement
- Letter from landlord giving you permission to live in the property when you arrive
Evidence that we have met/ongoing relationship:
- Photos of us together/professional engagement photos (no more than 2 photos in total)
- 3 Plane ticket/receipts of my travels to US to visit applicant
- 3 Plane ticket/receipts of applicant travelling to UK
- Plane tickets and hotel bookings of our travels together in US
- Print screen of emails received from applican
- Print screen of Skype conversation and calls
Evidence that we intend to marry:
- Booking of appointment to give notice to marry
- Confirmation of our marriage ceremony at local registry office
- Receipt of marriage ceremony booking
- Engagement ring receipt