DIY parts

[jhaible]

> I chose the integrator type of design for the following reasons. 

Good reasons indeed, and I won't question them.

But still, a few points to add:
(1)You could use a guard ring without a virtual gnd node, too.
Connect it to the buffer output instead.
(2) You can use an extra transistor for a cascode configuration
if you want to keep the cb voltage of the expo converter's transistor
low. This will also help making a high output impedance.
Korg used an extra FET for this cascode configuration in some of
their VCOs. (Expo converter from dual npn, output drives FET with
base grounded, discharge circuit is a thyristor built from discrete 
npn's and pnp's, and buffer is another FET. 
I do *not* know how good this performs, but I thought I should mention it.

JH.