Best choice to get data from third party system

Hi people.
I need your guide.

I always use sap connector 3.0 (c#) to get information from sap and it works good.

My concern is:

There are few third party systems (.NET c#) that require to get some information and take it to sap S4. (invoices, purchase orders, so on).
What is the best technical way to achieve this?

pd: NO XI is needed.

¿I use a web service to get the data from third party system?, then transform the documents in IDOC ?
¿Maybe sap connector 3.0 (.NET c#) to achieve this? .. i LIKE THIS..but i dont know how to configure the SM59. (Help in detail is needed here).

Please help to clear my mind and do the best job.
Regards.