The K 5 !
Man, That was my first big keyboard, I know you have the K5 module. It's true that it has a nice piano patch, that is what drew me to it. The main thing you have to remember, is you'll find the K5 on the "bright" and crisp sounding site sounding side. You won't be able to make "fat pad like" sounds on it, even the strtings and syth patchs sounds "digitally bright" It is not like the later made K1 or K1m, it is not a hybrid of additive synthesis and PCM sounds.
However you can beat the piano, brass, and rhodes sounds from the K5.
It's been a while, but Muti Mode is when you can select two or more differn't single sound patch's, I don't recall how, just that the keyboard is setup into 4 "zones", so you could have a bass patch sound on the left side of the keyboard and a piano patch sound on the right.
Maybe you can find a manual from someone here or on the internet. The last thing is, KEEP IT, don't trade it like I did, you'll regret it. I still have my K1m though. Nice!
good luck!
crush