Okay, I understand better now.
One problem I see which doesn't really effect your end goal, is that there is a lot of conversions between the analogue and digital realm, ADC and DAC.
As long as you know the details of each conversion process to know that you will have a faithful 4-20mA representation of final signal, which I guess will again go into another analogue to digital converter, such as PLC for further processing or to SCADA, correct ?
Would it not be possible to read the the sensor signal directly as analogue with as I mentioned using a quality ADC such as MCP3234 and then do all the processing you need using the Arduino board of choice or other micro-controller ?
If you are indeed sending to a device like a PLC, then why not send the processed data using digital, say, using Modbus, either Modbus TCP on Ethernet or Modbus RTU on TIA-485 or RS-232 ?
An Arduino Mega 2560 will be fine, but there is little cost difference between many of the boards.
You may be able to use a UNO style or go 1284 which is a sort of Goldilocks thing, or in fact a DUE, where you do have the on-board 12 bit ADC and DAC if you choose.
You mention multiple pressure sensors, what sort are they, do you have a part number ?
Just a few thoughts and methods I have used in the past.