I'd recommend the MKS-80. Unless you've got more money than you know what to do with you're probably better off at least starting with an MKS-80 and seeing where that leaves you.
It really only sells for unreasonable prices on eBay, and you can still get them well-priced locally, through the Analogue Heaven Mailing List or here on the Vintage Synth forums. If it turns out to not be your cup of tea you really shouldn't have a problem selling it for what you paid... but if you had to turn around and sell an Alesis Andromeda or Studio Electronic Omega8 because they didn't suit you it might be a little more difficult due to their higher costs.
The MKS-80 does have really good MIDI/SYSEX implementation as well, and if you need hands-on control but don't want to shell out the extra cash for an MPG-80 the Kiwitronics Patch Editor
covers the most important functions.
Personally I think most electronic musicians only need one or two good polyphonic synthesizers, as it's far more common to use monophonic instrument lines in most forms of music. Most of us have quite a few polyphonic synthesizers and are left lusting after more, and while I can't guarantee an MKS-80 will stop you from lusting after other synthesizers I can tell you it can act as a suitable substitute for quite a few of them. It has a good sound and it's unlikely you would disappointed with it.
It's worth noting the MKS-80 is a suitable candidate for a third-party upgrade as well. The VCO chips, in theory, can be configured to output all of their waveforms at once, rather than muting unused waveforms. An enhanced CPU may be able to add additional envelopes as well. I have no idea if anyone will ever
decide to tackle this niche and provide it as a kit to the public, but it bodes well for a synthesizer that is already pretty awesome to have the possibility
of having more life breathed in to it in the future.
You will most likely have questions about the MKS-80 rev4 and rev5, and the bottom line is it doesn't matter which revision you get. I have a rev4 and have only used a friend's rev5 sparingly, but someone I trust when it comes to electronics and synthesizers has done an extensive side-by-side comparison of the two. He concluded while there is a difference between the two, especially when loading the same patch in to rev4 and rev5 units, this can be remedied 99% of the time by adjusting the filter cutoff or resonance on the rev5 unit. Don't buy in to the hype about rev4 and rev5 units.