David
If you are talking about the Apple "iBook" proper, they were "entry level" devices that were discontinued in 2006. Even if you were to find a new one "in the box", I am afraid it would be a little too antiquated and would lack the processing power to handle today's music programs, particularly VST software like Pianoteq. If you are talking about small laptop type computers known generically as "netbooks", then that opens up an entirely different field. They come in a variety of configurations, all the way from a couple of hundred dollars to several hundred dollars, depending on processing power and features. Per Pianoteq's website, it will currently run on Mac OS X 10.5 or later, Windows XP/Vista/7/8, and Linux x86. It appears that they do not currently have a version that will run on the iOS of the iPad. If you are looking at the smaller units strictly for the convenience of size, that is one thing, but if you are doing that from the standpoint of lower cost, then you are most likely headed down the wrong path. Modern VST music programs like Pianoteq are some of the most taxing software that you can run on today's computer hardware. It is probably second only to high speed gaming programs, if even then. This is one case where "you get what you pay for" certainly applies. The Pianoteq website offers a trial download of their product, but since you are at the stage of selecting the hardware, it would probably be best to call them and ask them for recommendations, keeping in mind that meeting only the bare minimum hardware requirements for any given piece of software will most assuredly lead to disappointment.
Good luck !