Best Help Authoring Tool

I would appreciate any recommendation for a good help authoring tool.  I am new to this topic and am overwhelmed with all the help products and terminology (which I hope that I put this under the appropriate zone). Currently we are using Help and Manual 5 and 'publishing' as chm and pdf.
Basically what we have is 4 products which have similar contents and images.  Two go even further in that they are very customer specific; the same product with different clients.  They would like to make a change to one product 'help' and have it cascade/inherit to the others.  They do not want to make changes to each project.
 
Below are some specs that were requested:
- Screenshot repository
- Reuse of text
- Easy translation
- Customer specific
- Low risk (industry standard)
- Installed with software

Also could you please let me know the right terminology for my specs (and excuse any bad terminology I might have used).

Thanks in advance for any info&.