I might look at Autodesk's Fusion 360 which is fully Cloud Based. It should be released this year. All data is stored on the web with a local user interface app. As long as you have a internet connection you can log-in and work on the data stored within the Cloud.
All the "heavy lifting" is performed on the cloud so any Windows or Mac based PC can work on the largest of models. AU2012 they even hinted that tablets and iPads could/will be able to work on large assemblies. All that is required is a small user interface app that install on the local machine
With your description of local users and remote users and having problems to access your server along with the type of data transfer rates to connect and work remotely it would be tough to manage your data.
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Another choice would be a Wide Area File System (WAFS) product. These are software or hardware based solutions to accelerate the flow of information between sites. It could be as simple as mirroring directories between servers or as sophisticated as dedicated hardware using special algorithms to reduce the data flow across the WAN. There are dozens of solutions out there.
We use Riverbed (hardware) between our sites to accelerate data flow. It can often cut repeating data by 90%+. It only sends the data that has changed in the file. It has the ability to accelerate many kinds of data; CIFS (think windows file manager), Exchange and several other protocols on the network.