It will add load when searches or when reviewing. If you enable alalyitics it will cause a very heavy load on your EV server.

It is highly recommended that DA be in a server by it's self and also if you have a larger environment the DA SQL databases should be on their own SQL server seperate from EV

yep I've read that but what we are having issues with

is the customer wants too see metrics for the overhead

The metrics will all depend on each customers configuration and load. It is impossable to say that X% will be the overhead because the number of mailboxes being archived, if FSA is being run, if journaling is being done the number of DA searches and the size of the searches, if analytics will be used, how often legal holds will be put on and off.

All these will have an effect on the output and performance of EV and DA when on the same box.

Unless you have a small environment then i say no to DA on the same box.  

there is no FSA

It's running Journalling and has 160 mailboxes with just under 4 millions items archived

Well if they have good hardware you should get away with DA on the same server.

If you can upgrade the memory to greater than 4GB...recommended 8GB or more and make the OS 64Bit

What version of EV are you running.