The Jen SX1000 has been one of the most affordable analogue monosynths out there for many years. Where it's limited in features it makes up for in simplicity and raw power - it also features glorious pulse-width modulation! In terms of modding it's one of the simplest units out there to play with as the boards are split into modular parts - the filter, envelope, mixer, oscillator and power all separated.
Neil Johnson has a superb site on adding mods to the synth, including filter input, a second oscillator and now adding MIDI capability. He even did a talk about the mods for Anglia Ruskin University, which is available as a PDF.
There's also a very good Kontakt instrument made by Synth Magic which replicates the experience of owning one well, with some added features.