SQL Express will be fine -- and it is also easy to upgrade to SQL Server from SQL Express later (just use the SQL Management Studio from SQL Server to connect to SQL Express, once SQL Server is isntalled, and BACKUP THE DB to a file, DONT use Export/Import, and then RESTORE the db back to your new SQL server, then just change the server settings in LDMS to point to the real SQl server) I would advise against using Oracle as it is a lot more complicated, and not necessary for that few clients.
Essentially - what James said.
You can find good guidelines here in this doc:
In addition, while SQL Express/Server are reasonably easy to maintain even by people who haven't done too much with databases, Oracle is VERY different. Oracle is NOT something you do as an "occasional" thing on the side, and you'd need a full-time Oracle DBA for it, really.
So, for your environment, SQL Express is most likely the easiest choice, as has been mentioned - and if you grow out of it, it's pretty easy to upgrade to SQL Server as needed.
LANDesk EMEA Technical Lead.