One other option that I woulld consider at 500 USD.

Audiophile 24 / 96 $100 Usd

Used kb w/ midi - $50-100

USB's Plugsound series of modules at $300 Usd. These will cover every major sound category including acoustic and electronic drums. You'll wind up with 6 specialized modules that are wavetable based ( no analog modeling ) but the synth sounds are very well represented in module 5. I really like this for the price I paid.

Add a freeware Vsti host.. I'm sure you can find at least one suitable one here: http://www.kvraudio.com/get.php?mode=results&st=adv&soft=h&type%5B%5D=0&f=0&fe=vst&win=1&free=1&sf=0&receptor=&sort=1&rpp=30&sid=df62394c53d5df02f92f07cb3619c737

Then add Synth 1 for analog modeling along with some of the other freeware synths and effects I mentioned in my previous post. At 500, I doubt you'd ever come close sonically with a hardware board, even a used one.

Regards,
AJ