You will need to know things like:
1. Bandwidth requirements per user & per session.
2. Firewall configuration requirements to permit connection to their hosted services.
3. Backup & restore SLA’s
4. Availability SLA’s
5. Browser version requirements
6. Support agreement – calls permitted from all users or limited access; how easy is it to escalate support questions – pay lots of attention to this area as it could be your biggest challenge in implementing this new application.