If this is a dedicated server, Go ahead and install these.<br>Only thing that can be heavy is the Cron, as Apachesolr and Memcache have their mechanisms which might take long to execute.<br>For Apachesolr, You can use jetty, which is much lighter than Tomcat, and a better option for me.<br>

<br>Also, Varnish wont be good to implement if you have authenticated users much more than Anonymous users.<br>Use varnish only in case you see no of hits are more by anonymous users, Like in case of news / blog sites.<br>