620 is a very good starting keyboard with some professional features like DSP processor. Sound qulality is very good even styles are great and of course you can take advantage of net to find and download free styles and midi songs to use with 620. But it has hosted a DD-720KB disk drive so it might be difficult to buy 720's now a days. But you can use 2HD by a little modification (not tested).

620 'd cost you around $320 in a good condition.