Hi Sudhakar,
You can perform from production system to test environment (Either system copy or clone the production to private environment)and then perform the upgrade task to ECC6 EHP7 by using SUM tool, calculate the over all time(Tech + Business Down time). this will give you complete overview of the downtime required in production environment.
NZDT is service provided by SAP exclusively as paid service. Indeed it can bring downtime as much as minimum level possible.
Regards
Ram