Approach to Handle External Application with SAP Business One

Dear Experts,

I am keeping a Central System as SAP Business One ,( Backend MS SQL Server) where my Master get created. (such as Item Master , BP Master,Price List, WareHouse).

Now I have third party Dot net based application POS system (Backend MS SQL Server) which requires to access the Master Data from SAP  server . Later need to push the Transaction Data customer wise to SAP Database. Per day 20,000 customer transaction takes place.

Also Web application need to push the data to SAP system.

Three option are available DIAPI, B1isn , B1if.

I suggested to build the third party Dot Net application to build as Addon based  but the Database will be in their own server . I required the type of License required to communicate between the two server (Note: third party application user login are upto 100 Nos).

I required useful information, guidance and source for the following Business Process in Entertainment Park.

Regards:

Prasanna