Development of a "secure" web portal

Question

I need to develop a web portal, where both lawyers and agents access to execute legal transactions. The idea is that agents and lawyers register on the portal. The agents, once registered can open new cases that will have to be picked up by some lawyers for review. So payments will have to be made by agent when a lawyer accepts to take over his or her case. Therefore, the case goes through some steps using a workflow (new -> under review -> accepted/reject -> paid etc.)

 

The portal has to be available in 4 languages, and it is of great importance that all transactions are encrypted.

 

I need to know please ASAP:

1- What would be the best language/plaform/framework (.net, java, Node.js, sharepoint etc.) to build such a secure portal?

2- What would be the best way to secure messages between agents and lawyers.