Hi
I thing your migration are in two different phases
Phase 1. With Current SAP level to minimum supported Windows OS & MS SQl server with MSCS (May be on Windows 2003 with MS Sql)
SAP Current release to ECC 6 With NW 7.00 SPS level 20
Phase 2. Migrate the OS /DB either Windows 2008 R2 or 2012 R2 / DB MS SQl 2008 R2 or 2012 R2 and once again perform upgrade from ECC6 with NW 7.0(No EHP) to latest version ECC 6 EHP6 with NW 7.3 or 7 with NW 7.4,
1076022 - Release planning for Microsoft SQL Server 2008 (R2)
1651862 - Release planning for Microsoft SQL Server 2012
Pl check the PAM http://service.sap.com/pam
3. In case your current system connected the any other SAP modules you have to refer the SAP Note
1388258 - Version Interoperability within the SAP Business Suite
Regards
Sriram