Can you install Quickbooks on a Linux/Apache environment? Will it work?
No, it will not work, quickbooks runs as a standalone app that can be networked, this requires windows, and this guy will need a dedicated environment.
You will need about 1mbit of bandwidth up/down for it to run with 5 users, however the bandwidth depends on how many transactions they run, usually the client downloads the .qb file from the server then uploads it back when its done.
Ofcourse this depends on how they want to use this, another option is to setup vnc or windows terminal services with 5-8 users and give each access to Quickbooks.