I've not seen the equivalent of Notes' views in Alfresco. However there's are view templates to control which properties are shown. And there's categorisation and tagging. But the alfresco interface tends to be search driven rather than having a set of pre-defined "views" like Notes.
The second thing that is always tricky, regardless of system, is to pull in keyword data from outside the system. Some sort of ETL solution would probably be required. But the idea is to take a copy of the keyword data and put it into Alfresco or Notes.
However now onto some of the things that you can do
You can build an application front end in the technology of your choice and use Alfresco as a "back end".
You could model your types for clients and companies and create associations between the documents and the clients and companies. You can make this as complicated as you like or as simple as a folder for each client or company.
You also have extensive metadata and security capabilities in alfresco.
Your first point of call is to understand Alfresco. Then you need to decide whether you need to write your own UI to supplement those provided by Alfresco.
The second thing that is always tricky, regardless of system, is to pull in keyword data from outside the system. Some sort of ETL solution would probably be required. But the idea is to take a copy of the keyword data and put it into Alfresco or Notes.
However now onto some of the things that you can do
You can build an application front end in the technology of your choice and use Alfresco as a "back end".
You could model your types for clients and companies and create associations between the documents and the clients and companies. You can make this as complicated as you like or as simple as a folder for each client or company.
You also have extensive metadata and security capabilities in alfresco.
Your first point of call is to understand Alfresco. Then you need to decide whether you need to write your own UI to supplement those provided by Alfresco.
I'm a server-side developer so don't often get let loose on UI design. In fact the Alfresco UI team very politely told me to stop touching their code yesterday. 
It rather depends upon your requirements and what technologies it needs to fit into.
I'm presuming you want some sort of browser based application then.
I'd certainly try the Alfresco Surf framework which looks like it has great potential for something much bigger than even Alfresco.
I like JSF (although not the way Alfresco has used it).
And I'd possibly use Visual Studio as well.
It rather depends upon your requirements and what technologies it needs to fit into.
I'm presuming you want some sort of browser based application then.
I'd certainly try the Alfresco Surf framework which looks like it has great potential for something much bigger than even Alfresco.
I like JSF (although not the way Alfresco has used it).
And I'd possibly use Visual Studio as well.
