RS-485 Modbus device to MCU
Yes, obviously you need an UART. Your driver looks also okay. One UART is enough. Do not attempt to build your project when you are not familiar with terms like: Bus system, RS485, Physical Layer, UART, RS232, half duplex, Master-Slave topology, bus-arbitration. With that you will understand what a bus system is and how multiple devices on a single line work. A good starting point are application notes from some rs485 vendors and tutorials, for example this one. 

Personally, I used the ST3485, which has an application note explaining some basic stuff here but you can find the same information from all other vendors also. 

For your Firmware and Protocool decryption there are many librarys out there. First of all you have to know if you want to build a Master or a Slave device. For all of the above: Google is your friend. But for google to find the right stuff, you need to know the terminology (what to search)!