The unit shall receive the MIDI signal and physically move an (audio)
potentiometer, I know that you can buy a readymade MIDI mixer but this
is another project.

Do you have any tips?

Stefan Consider the dallas pots use a '54 to recive the MIDI and then control
the DS1628 (i think data at work) these are auido grade digital pots with a
up and down input lot easyer than motors and pots due to being digital they
dont get noisy wit age and lack of lubrication

4 chip soulition
1. CNY17 or HP equiv for MIDI isolation
2. PIC16C54 or 12C508 Brain
3. DS1628 digital pot there is a stereo version but cant remember pt number
4. NE5532 / TL072 for signal conditioning the audio

Total componant count less than 30 will run on battries no moving parts to
wear out if using a '54 the entire B port could be dedicated to a channel
select switch array for selective opperation could use loads of these on one
MIDI controler.

Just an idea Steve..........