As you know this is an arranger forum and I think (and many pro's here also) that arranger is a great tool to use for beginner and people who wants to play almost any kind of music and needs accompaniment.

If you just want piano and strings, many arrangers have decent ones although I am sure dedicated synths and piano boards are probably better sounding. Less than $400, for new arranger board has got to be the CASIO WK3000 or the 3500 series. But for a used board, maybe Yamaha PSR740 or PSR550 or even PSR1000.