You could try one of the higher end Yamaha PSRs with XG. XG level 1 has 480 instruments including a lot of sampled strings and the PSRs have extra sounds as well. My PSR740 has 761 instruments altogether and I can layer up to 18 at once. I'm currently using some nice organ sounds with 6 layers. The nice thing about this range of models is that you don't need a sound card. I use a "to host" connection to my laptop which is simply a cable that plugs into a serial port. The cable is cheap or you can make one yourself as I did. Or you can use the USB port with a midi adapter thopugh later PSRs also have a USB port. Software that utilises XG includes XGWorks and XGEdit. New PSR740s were recently being sold for as low as $600 in some places due to them being discontinued.

Bryan