Whoa! Not so fast. At that age it will have Roland's SA synthesis sound engine. Not very piano like but amazingly expressive...much more so than the vast majority of brand new DPs in fact. The SA sounds are completely modelled. Legend has it that Roland had to abandon the technology because the inventor died.
On the other hand it may be a break-down waiting to happen so, yes, worth having, but only if it's in full working order and very cheap...that means no more than £200ish...what's that in dollars? $300ish I guess. Don't pay more!
Yes Steve, SAS was the top technology for Roland back then...Structured Adaptive Synthesis it stood for...I have it in my old HP-1700 and it is very expressive, although the mids are a little thin...much like today's Rolands.
The SAS was also used to great success in Roland's VK-1000 drawbar organ (I had one for awhile).
The HP-3500 was fairly near the top of the range back then...around 1989, I believe, it was the bottom model of an upper end group which consisted of HP-5000/HP-4000L/HP-3500...I believe they all had MIDI in/out, and all used the SK-6 action, which was weighted rotary oil damped, and felt terrific, as it does on my HP-1700.
I'm to understand that SAS involved the taking apart, examination, and reassembly of sound participles, with respect to amplification, MIDI velocity sensitivity, etc. in order to inject some life into the then fossilized world of digital pianos.
As you say. it may be worth it...if the price is right.