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The VBM has a bit more grunt than the Silvia as it has a larger boiler. It also has better temperature stability unless you get the Silvia + PID
How good the VBM is now depends on how well it was looked after when in constant use, and what state it was left in when not (eg did they empty the boiler before storing)
I would be getting the current owner to fire it up and make some coffee from it.
How much are they asking? I would figure no more than $1000 for something of this vintage unless it is in as-new condition. You can pick up a brand new Rocky for less than this (one sponsor has a Silvia on special at <$800)
Another option at the sub-$1K price point is the GEE that is pretty impressive from the demo I saw last year. A couple of sponsors are selling.