My guitar processor "Muffy" can be seen at http://www.wiseguysynth.com/larry/guitar.htm
It is essentially a hex fuzz outputting quasi-sawtooth, square, pulse, and rectified pulse (gives a sort of octave up effect). It has six envelope followers
controlling six 12dB VCFs with resonance. It has six VCA's which can act as either gates of be controlled by the envelope, or bypassed. Obviously it cannot
sustain more than the string itself does. There is an attack-delay circuit for each envelope follower. There are no 'unobtanium' chips used.
If I were doing it again, I'd make a voltage controlled ADS generator (you really can't have 'release' but what I've designed is a unit that shares the decay and release times. Careful setting of a trigger level could give access to all sorts of percussive sounds.
The actual P/V converter I added is about as complex as one of these boards, and that just gets you three strings monophonic. The best way to really get guitar synth imho would be a board per string, with P/V conversion, direct waveform synthesis from the guitar string, and an envelope follower / ADS generator, VCF and VCA. I can generate octave down square/sawtooth, fundamental square and sawtooth, (ready for this one ?) fifth up sawtooth, and octave up sawtooth from the guitar string directly. I'd include a VCO with saw, triangle, and pulse waves that can be hard sync'd to the guitar waveform (this eliminates any pitch tracking delays, instead you get a waveform distortion which is not very noticible... or you can have the VCO and guitar wave ~not~ sync'd, that allows the direct guitar wave to cover up the inevitable delay in P/V tracking. My P/V converter gets an output within one cycle of the fundamental so its maybe 2X faster than most commercial units... but it is complex. I also have a circuit that eliminates the droop when you lift your finger from a string. This makes the note go flat as a rule, as your finger does not damp the string in time to prevent converting that now slightly longer string length. P/V conversion is kind of nasty about these things...
Downside, connecting six boards to a voltage controlled front panel and putting it in a box is not cheap. The overall complexity would probably be about the same as an all-analog polysynth, without the neat SSM or CEM chips to do the functions. Probably it would be like an olf Oberheim 8 voice polystnth (made from SEM modules) in complexity. NOT for the faint of heart, eh
You could buy a Roland V-guitar for the price. Or any vintage guitar synth, but at least this one would work...
Maybe I'll get the camera out a photo what I have now...