First of all you should realize that the question "What hardware do I need for N concurrent users?" has no serious answer. Ref. https://docs.moodle.org/en/Performance_FAQ#What_hardware_should_I_buy_for_.27n.27_concurrent_users.3F
. What you could get here is to listen to other people's experiences. But there are just too many parameters even to count, you need to be very lucky to find somebody who has a comparable set up and ready to run a test for you and submit the results. So the most reliable thing in your case, just a single dedicated computer, running (RedHat?) Linux, is to do a test run. Installing LAMP
and Moodle doesn't take much time.
I am curious though. You have a web cluster running Moodle already. Why another computer, just for the exams?
And what is this "server". 2.2 GHz CPU (single core?) and 2 GB RAM sound more like a disposed PC. The lowest CPU has 4 cores or 4 threads nowadays. And RAM is cheap, 8 GB costs peanuts. That said, I can not believe yours is single core. Run 'less /proc/cpuinfo' to check.
The general experience here is that upto about 30 to 50 quiz users a common machine with good disk system and lot of RAM is sufficient. For those "about 70" things could get critical. Do all of them start the on-line exam in the same minute, for example? Or is the full one hour exam with 60 questions is in one page? These thing have big effect on the server load.