It sounds as if you need quite an advanced software, there's quite a lot of software about which are completely online and probably wouldn't require you to modify it.
If you you're mainly focussing on files rather than discussions/task manager/event manger/pages functionality, It sounds like the Clinked SmartDrive
would be better (and cheaper) for you in terms of the requirements that you've mentioned. SmartDrive has the option to be used online, but also has a handy SmartSync
desktop program/application. It's all secure and has some great functions such as version history and other collaboration tools which makes it stand out from the other applications.Clinked
has all of the details.
I work for Clinked so if I can help you at all, just ask!