I know this is a late reply, but you can find a Moog Minitaur on Ebay for under $400. It will not make sounds more than an octave above middle C. If all you want is lower frequency it doesn't have many rivals. You could also score a Waldorf Pulse 2 for about $500 on Ebay, which is more versatile and still a great synth. Both of these synths would be a purchase that could last the rest of your life. Not sure how the build quality of the Pulse 2 would last in 20 years time as it has a display. The Minitaur will become a classic, it has a great Moog sound and a solid build quality that should stand up to the test of time.
You should listen to both before making a decision.
Minitaur
http://www.youtube.com/watch?v=XS1KwHv26uoPulse 2
http://www.youtube.com/watch?v=7HIUkU4UbJcAlso check out KayKayOnTheBeatz as she uses both for hip hop.
https://www.youtube.com/user/KayKayONTHEBEATZ