That's a big project I guess, sensing the strings is a big deal, the strings which are always open is not a big problem since you only need to see if they are playing and maybe the sensitivity, so note on message and level, not a problem, takes no time. The problem is with the other strings, when they may be different notes, so you need to detect that note. I don't know the actual processing normally used to do that but it must be fast and is not easy to detect a note from the first few ms of it. You will need a good processing power to do that as well. Once you have the data you need to transfer via MIDI is pretty straight forward, a small arduino could do that. A bigger Arduino could do that plus the actuators and trigger management. The problem is the other part, you need 6 decent AD converter for each string plus one simpler AD for each open string to sense the level.
I don't know the usual hardware and software for each task, I guess a decent processor and a micro controller could manage all that stuff, raspberry pi, arduino, pic, comes to mind. There is a mixed arduino board which may do both things but you probably run out of pins pretty quickly from it, using a bigger arduino like a mega or so plus a raspberry is probably a better bet.
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