I have three clients (100-200 employees) that each run their entire business except for an accounting package using Access applications.
If you don't have any development skills in any other environment, it will take you quite a while to learn the new environment and get up to speed. Even still, coding everything from scratch is a lot more work than using a RAD tool.
Divide and conquer. There is no reason to not create the back office application using Access which you claim to be fluent with. Then you can find a tool to use for the portal.