obsolete processor

You can use any controller of your choice. But before selecting any controller, List down all the requirements for your project.
List down all the hardware requirements viz total number of timers, uarts, gpio lines, memory requirements (code and data memories) etc.
Also decide if you will interface a 16*2 character LCD, graphic LCD or TFT or ...
Do you need a touch interface?
SD card, USB, ethernet interface, etc.

All this will in turn help you to decide whether you are going to use procedural C or RTOS or any other OS for development.

Finally select a architecture and hence the IC.

You can use any controller of your choice. But before selecting any controller, List down all the requirements for your project.
List down all the hardware requirements viz total number of timers, uarts, gpio lines, memory requirements (code and data memories) etc.
Also decide if you will interface a 16*2 character LCD, graphic LCD or TFT or ...
Do you need a touch interface?
SD card, USB, ethernet interface, etc.

All this will in turn help you to decide whether you are going to use procedural C or RTOS or any other OS for development.

Finally select a architecture and hence the IC.

That's not an either/or choice!

You can use procedural 'C' with or without an (RT)OS;

You can use an (RT)OS with or without procedural 'C'.