We are adding a search head server and I am trying to work out how much HDD space will be required. My understanding is that indexers require the largest amount of HDD space as they index and store the data. What about a search head? We are planning to run a lot of scheduled searches and summary indexes.
The number of searches that you run does affect the disk space on a search head. The results of searches are stored in $SPLUNK_HOME/var/run/splunk/dispatch
You could look at your existing servers to see how much disk space this requires. It is probably tiny compared to your indexes...
On a search head, I usually set up a dedicated drive or mount point for the $SPLUNK_HOME/var directory tree. That way it is easy to monitor. The var subdirectory contains all of the "dynamic" files that are created: log files, search results, etc.