SRM-MDM dont exist. Really it is MDM CATALOG 3.0
You can buy this sap´s product with out SRM. Because MDM CATALOG is a simple repository into a database on Oracle (by example), and SAP give you a content with a repository created about the catalogs.
The front end for MDM CATALOG is a web service that run in JAVA SERVER. So you can conect ECC with MDM catalog. ECC consume this services. maybe you will need prepare any services or badi to consume this data (transfer from WEB services - to ECC)
tecnically you havent problem, but. i dont know if SAP have any consideration about the license.
In other hand, you can buy the MDM 7.0, is the solution from SAP to maintain the master data, MDM 7.0 (import,console, data manager, SYNDICATOR, PUBLISHER) is more robust that MDM3.0 (import,console, data manager)
When you have the MDM 7.0, you install the content to catalog, and ready, you have the repositroy ready to work by catalogs.
i share you a link with information about MDM-CATALOG.
SAP NetWeaver Master Data Management - Animation
regards