Building a client portal with Sharepoint

I'm really new to Sharepoint and I would like to ask our case if this is even possible. We wanted to build a custom client/billing portal thru Sharepoint. Can this be possible or there's going to be a lot of work involved?

Users will probably have O365 logins to access this site or will be given access (probably 1 for each client).

I know this might be a lot for you to respond but any insights or links where I can read and move forward is highly appreciated.