>>My understanding is that for single user Express is free and would do the job? after that it's Workgroup? Is that OK for up to 5 users?

The free version will probably work for you, if your DB stays under the 4GB limit.
Check the MS web site for other limitations.
I got started with the Enterprise version (pricey) and never really looked at the free version because of that limitation, but it's probably a good choice in many cases.