Hi Jan,
Suggestions for additional CPU, RAM will be depend on your existing Server configuration.
In case there are no performance bottlenecks in your existing system, there is no need for additional CPU and RAM during upgrade to BI 7.3 NUC.
Secondly there will be requirement of additional disk space as a new tablespace will be get created for the new release. Assume a rough estimation of 20% increase in existing DB size during upgrade to BI 7.3 NUC. So You may need to plan the storage space accordingly.
Apart from that for upgrade directory and DVD dumps you may need to provision some storage space which should be rought around 30 GB in all.
Hope this helps.
Regards,
Deepak Kori