Monitoring of a System through an Arduino?
You need to measure 18 analog signals, and even a Mega has only 16 analog inputs. If you settle for 16 channels, consider that arduinos have a single ADC, so your measurements will be taken sequentially and shifted in time by about 0.1 ms. This means a delay of 1.6 ms between the first and the last measurement in a sample.

If you're OK with the limitations mentioned above, you could use an arduino for this task. However, the professional thing would be to buy a data logger which has enough channels, much better precision, synchronous samples, and most importantly works out of the box. You will save a lot of time and money to the project by using a reliable equipment you don't have to debug and calibrate.