For the best chance of success these three should be prioritized and the project plan should be based on these priorities so that if estimates are not met then the hit will be taken in the less critical areas.
People & Companies Involved
General Activities to be Assigned
Specification & Requirements
Design
Development Planning
Management & Tracking
Implementation (Applications, Reports, etc)
Testing & Qualification
Production Deployment and Maintenance
Issue Research & Resolution
End-user & Technical Training
End-user & Technical Support
End-user Company
Desired level of involvement
Expertise available for project
Feelings about open source software and proprietary versus open source results
Service Providers
Existing and/or trusted third party service providers
Initial Design, Development, Training, et cetera
Long Term Services (such as hosting, technical support, etc)
Contract Structure
Fixed rate service over time (like hosting, some types of support)
Fixed rate for a one-time fixed service (fixed bid)
Hourly service, to make sure services are available may purchase hours in advance or agree on a certain number of hours over a period of time
Progressive fixed-bid, perhaps with a contract for work over a certain period of time (works well with Agile and XP methodologies)
When using one or more service providers make sure it is understood who will do what, and who is responsible for unplanned needs that come up. For example, is the solution provider offering a "complete package", and what does that mean, or a only specific functionality and services?
Load & Scale Planning
Visits & Page Hits
Typical & Peak Loads
Estimate periodic peaks using lower numbers of time periods per day, for example 14 hours per day; these should be based on (if available) historic load distributions
Orders, Customer Service Calls, Work Efforts, Catalog Changes, and so on
Total Products, Customers, etc
HTML & Image sizing, bandwidth use
Use these for a rough estimate and as a basis for the pre-production load tests
General Issues
Business Process Analysis
Existing Processes
Desired Processes
Design & Optimize Processes
Based on Physical and Business Constraints
Based on Software Constraints
As Preparation for Custom Software
User Interface Design & Specifications (including administrative and customer facing)
Reports: Existing, Necessary, and Desired (with parameters, specific fields, etc)
System Analysis & Division of Labor
Old Systems to be Replaced
Migrate data to new or existing systems
What did the system do, and what will replace it?
Existing System to be Kept
Integrate with new or existing systems
What will these systems do and what will new system(s) do?
Data Migrated to or from the System
New System(s) to be Added
Data Migrated to the System
Integration with Existing Systems
Location and Responsibility for Systems
End-user Internal
May be ERP, fulfillment, CRM, reporting, system monitoring and many other types of systems
Hosted on Behalf of End-user
Common for eCommerce, System Monitoring
Sometimes used (especially with web based applications) for other types, including ERP, CRM, reporting, etc
Third Party Systems
Common for Payment Processing, System Montoring
Sometimes used for General Content, Catalog/Product Information, Shipping Calculation, reporting, and more
Security
Network configuration and systems access
Encryption
SSL Certificates and other web-based encryption issues
Email Certificates
Planning Roles and Permissions for all users and system functions
eCommerce and Retail Specific Issues
Product Information
Finding Products
Browsing & Categorization
Special Categories (promotions, top 10, new products, etc)
By product type
By target audience
Number of tree levels
Searching in Categories
Searching (keyword, parametric, in categories, sorting, etc)
Describing Products
Variants
Product Content
Name, Short & Long Description
Images
Special Content (warnings, directions, etc)
Cross- & Up-Sells, and other Product Associations
Product Features (size, color, brand, ingredients, packaging, material, functions, etc)
Pricing
Type of Product Prices
Price Rules
Promotions
Promotion Rules
Promotion Codes
Use Limits
Lots of options!
Customer Information
Profile
Personal Information (name, birthdate, gender, etc, etc)