I would not focus on or specifiy "legal work" per se.
Use your specific security requirements as a basis for selecting the collaboration software.
Start with the lawyer's themselves - what collaboration software do they use now and recommend?
Remember that many such applications may allow you to have a free trial period to use and test the software.
If so, put together an evaluation plan so you can try, test, and see how products compare and perform in both form and function.
How User friendly? What features and functions are available?
Hardware support requirements, costs (initial and ongoing). Host computers, networking, communications requirements?
Just my thoughts on the matter. There may be other ideas and suggestions.