i found myself in the same position some month ago,
my wife did her license (A2 - bikes with a limit of 35kW/47bhp) a month ago,
so we wanted wo buy a 250-300cc bike.
Most of the used 250 bikes are "supersport"-like bikes,
but she never felt comfortable on them.
So the options are z300 (new), Duke 390 (used, 2years), cbf500 (new).
She's not a honda fan (but loves marquez), so no honda.
The seat position on the Duke was way to high, as was the price.
Looked for a presentation model z300 and found one 2 hours away.
I bought the presentation model (~45km) for 700€ less than the new price.
She loves her z300, the bike has more power as a 250 and is so easy to ride.
My wife is 162cm tall and slim, so it was important to find a bike that steers easily,
older bikes tend to be heavier and its harder to push it around.
If your wife feels comfortable on a ninja 300 or 250, i would buy a used one or a presentation model. i would prefer the 300, if you ride together its way easier for ther to follow you, cause the bike offers more power, with a smooth power curve.
if your wife really loves to ride and cares for the bike, try to ride as safe as possible and ride good i would also consider to buy a new one, if she just ride and doesnt care much i would buy a used one........ or i wouldnt buy one.