I owned the XD9 and sold it for the SD5. The xd9 is a great board, many of the xd9 styles are on the sd5, so your not missing much by going with the xd9. I sure that you can find one for $1000 or less (used). if you want to spend the money, get the SD5. I would not spend my money on a SD1 since the board is now 6-7 years old. The xd9 has great styles, voices, hard drive, vocalizer. It's lighter then the sd5.
If your looking for a ketron you can go wrong with the xd9, if you want to wait of audya or sd5, there great also but a lot more expensive.
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Ketron X1 (Oldie but Goodie)