I do not see an easy sollution for C++.
(Certainly not a cheap one)

When C can do, try ZWORLD's TCP/IP board, based on Rabbit's Z180 improved
chip.

It is low cost. ($199)

It has 10 Mbit UTP ethernet, RS232
AND RS485, plus some I/O lines.
It can do SPI. It has enough memory for you.

www.zworld.com
www.rabbitsemiconductor.com

(See also their new Intellicom)

It is good documented, contains a full
development / debugging chain.

If you go this way, please let me know
later on how you are doing.
It's always nice to share some experience.

Frank