RM1X comes to mind. they sometimes have malfunctioning buttons or knobs the sellers cant repair. So they sell them for around 150 €. currently there is such thing for sale on ebay Kleinanzeigen in Germany. He sells it as malfunctioning for 175 €. i had an rm1x once and as far as i remember it wasnt that hard to “repair” these things. buttons are often just malfunctioning because the terminal pad has dirt on it. or its corroded due to humidity. this is easy to fix. Other than that theres an ES-1 for sale on ebay Kleinanzeigen for 170 € at the moment. seems to be a good deal, considering that this is already a sampler.
i would not recommend him/her to get these lame apps. a tablet is not an instrument and will never be. its just a computer. it doesnt have a soul and a feeling to it. thats just simply it! scratch that really quick!
best advice would be however: he/she should save a little more and go into the market with around 300 €. for this money you can get some of the classics quite easily and he/she will definitely have much more fun with these.