User Management for e-Commerce ERP

Hello Experts,

I am working on e-Commerce ECC6.0 version.

Had some questions in the area of user management:

A)

The scenario is that the e-commerce application needs to be integrated to Poral, now my doubt is which userType option is the best to be chosen for such cases.

Is it mandatory to use user type "ERP e-Commerce logon via UME"?

I have not worked with UME before and hence if you can also share some links / pointers in that area?

what other SSO logon options are available?

B) In case there is no portal and e-Commerce is working as standalone application:

There is already a ECC6.0 system in place where until now customers have been maintained. The client has been using a custom Online Store application on which login was via the customer id. Now they want to use e-Commerce for ERP, but still want the logon to be via the customer id only. In such case the only option that I can think of is via "R3_SU05Customer_LoginCustomerNo" userType. But since su05 type is not recommended now, I was wondering if there could be some other option as well? Can you guide..

Thanks