At 150 US$ you can´t do wrong... if the unit is in working shape, that is.
I used to have a Compact Duo, and it was a nightmare to maintain after 35 or so years. Worst bit were the keyboard contacts of the upper manual which tend to corrode rather quickly, and it´s nearly impossible to dismantle the keyboard assembly to clean each single one of them. Also, some of the transistors on the voice boards seem to be prone to failure, and tuning the voices can be a bit of a headache as well because the coil trimmers may just crumble apart. When buying a Compact Duo, make sure the external FA/R preamp/spring reverb is included as these seem to have gone walkabout more often than once (I´m not sure whether the organ is playable without one). I sold the Compact Duo (as well as my red Compact with valve preamp) after I had them both fixed.
The VIP-600 is equally great, and if it was good enough for Edgar Froese (Tangerine Dream) or Cluster, it should be good enough for you as well
. The sound is a bit more modern and a bit more advanced but there is plenty of outboard stuff to make it sound dated.
The Professional Duo would be the be-all and end-all of Farfisas, I reckon.
"Like the light from distant stars, Stephen Parsick's music has existed for some time, but is only now reaching us on Earth." -- Chuck van Zyl / Star´s End