1) Output 10 volts mean that the PLC has to have analog output 0-10V. Only the T100MD1616+ has an analog output that can drive 0-10V load.
2) RS485 is communication interface and has nothing to do with the analog output. RS485 does not need 10V output. All our M-series PLC have RS485 so any model can use this method to interface to the servo motor.
3) You need to get the servo motor's communication protocol and then write a custom function to output the needed command strings to talk to the SERVO motor. Most likely you will use the NETCMD$ command to talk to the servo motor controller. There are other command available to talk to 3rd party devices: PRINT #, INPUT$, ?INCOMM, OUTCOMM.
4) You cannot use READMODBUS or WRITEMODBUS commands unless your servo motor support the MODBUS protocols.
5) There is no need to connect the PLC to the PC's modem. You just need a serial cable between the PLC and the PC. The PC will need to run the TLServer and have an internet connection. The PC's internet connection can be by any method (dialup, LAN, Wifi. whatever). Once the PC is connected to the internet the PLC become accessible by TRiLOGI client via the Internet.