I can't think of any decent used synths (with good sequencers) for $300. In the arranger market you're looking at a Casio WK-3200 for that price. The 3200 sounds great. It replaced the WK-3000 which also sounded good. The 3200 has new improved voice quality (along with improved drums). It also has twice the memory for user samples. This is actually an (under-rated) feature on the Casio. It allows you up to 4mb for user samples (you can download them into the keyboard by using the free software Casio has that converts them to Casio format).

Squeak
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GEAR: Yamaha MOXF-6, Casio MZX-500, Roland Juno-Di, M-Audio Venom, Roland RS-70, Yamaha PSR S700, M-Audio Axiom Pro-61 (Midi Controller). SOFTWARE: Mixcraft-7, PowerTracks Pro Audio 2013, Beat Thang Virtual, Dimension Le.