Buil SCADA: use Arduino+sensor to replace flow-meter & simple function PLC

Hello everyone,
I am doing my internship in a company in India. We have built a SCADA system by using GENESIS 32 software solution supplied by ICONICS.

First, I would like to talk about using Arduino to do some simple calculation (i.e: daily average consumption of Light Diesel Oil in my plant). We are using Rosemount Radar 5301 to measure level, volume & level rate of oil. By using log-tag, data can be recorded in server and presented in trend. What we want is to present the value of average daily consumption (level rate will be used to make the calculation) of last week in graphic screen (the screen displays the live data). But there is NO average function in Expression Editor (as showed in the picture below) and the software can not automatically pick the start & end point to implement the algorithm. I am suggested to use a PLC 8 I/O ports to work out this issue.

For more details, sensor uses RS485 output (most of equipments here use RS485 protocol), connect to Ethernet converter in control room, go to switch hub then go to server.

Second, we are considering to purchase some flow & level meter (for water and LDO) with accepted tolerance <5%. We received quotation from E+H & Emerson so far.

I would like to ask is it possible to use Arduino in first case? There are 2 points I doubt here: RS485 & OPC. As I know, I have to use RS485 shield as input pins of Arduino to receive data from Rosemount Sensor and use Ethernet shield to connect with server. About OPC server, do I need to configure/declare any thing or just simply connect Arduino to server then pick the data (tag) as the picture below?

For the second case, I found the meters is really expensive. We just need it to know the consumption in a specific time. Is it reliable when I use Arduino + mass flow sensor/ultrasonic sensor (we prefer mass flow & ultrasonic) to carry it out? In that case, Arduino would be located in control room so the distance to the sensor can go up to 100m. The architecture remains as above: sensor (output RS485) - arduino (output ethernet) - server.

I'm a newbie in Arduino and just do some basic tutorials. Maybe I have dumb questions, but it's meaningful to me. I tried to google before asking here but I feel lost in a sea of information. So please forgive me if you feel annoy about these questions.

Thank you in advance. I appreciate all of your helps.

Were you told to look for alternate solutions using an Arduino? Specifying Rosemount or Emerson sensors and then using Arduino to process them is, well, an uncommon thing to do and generally avoided for good reason. The flow sensors may seem expensive to you, but what's the cost of failure.

As for your problem computing averages in Genesis, I can't answer that, but it seems unlikely that it's impossible. Instead of jumping on an Arduino solution, ask the question at this site
It's the best PLC site I know of and the guys there are very knowledgeable and helpful.