I'd go with an ATmega 2560 and forget about anything much smaller. The cost difference is minimal and you'll save a lot of grief having the memory to spare and not having to struggle to source an Arduino based on a non-standard uPC.
If you're looking for something close, with Modbus for the interface, drop me a PM. I don't have the RS-485 interface done, but there are plenty of RS-232 to RS-485 converters on the market. Or if you have a few weeks to spare, I'll have the RS-485 driver laid out