Document sync / hosting system for field workers

Hey!

I'm in a situation where I need to go back to scratch and try to find a good document sync/backup/hosting system for field workers, that ain't cloud based (so no Dropbox etc).
So I'm kinda curious if some of you experts know a good way to achieve this, based on the premises below.

So, here's the deal:

This is a company where they go out and do building inspections like ventilation and testing humidity levels etc. They have about 50 employees that does this.
They have notebooks or pads out on the field that they use to write documents out on the field. They also take a lot of pictures.

As soon as they go online, they need to have their documents synced to a centralized server. This is because they need a centralized backup and the ability to get a hold of documents from whereever they are. This means that they also need to reach these documents as soon as they're uploaded. They also need to have documents synced back to them, if one of the receptionists place a document in their centralized folder. This implies a two way sync.
They also need group based security, meaning that certain groups of people can only access their own group's documents. Or even down to individual security.

Last but not least - they're producing a vast number of documents and pictures (like 120 000 a year), meaning that Sharepoint Workspace doesn't work.

This is how I solved it so far:

Sharepoint 2010 + Allway Sync (www.allwaysync.com) over WebDAV + adding a network share to "http://server.xxx.xxx/bla bla"

But there's so many problems with this, like the sync software sometimes fails, slow sharepoint unless you optimize the computers manually (turning off automatic proxy detection for example) and having the enter password everytime they access the network share after a fresh boot.

I solved the problem with constant password entry when opening documents by adding the computers to the domain, but they still don't remember the network share password between reboots.

The system is just.... not reliable, and I know Sharepoint ain't really meant to be used like this.


So my question to you - is there a good alternative for this? A document/picture hosting server software with sync capabilities for a vast number of documents from several clients, with at least group based securities. Oh and of course, as always, it needs to be "cost effective"..... ;)

Thank you in advance! I know my description of the problem may be a bit... chaotic, but I hope you understand what I mean.