Hi there,
I have an ER5 currently brilliant bike and as you already stated vastly underated. I picked it up in July for 700measly quid a 1999 model with 20k on the clock. Its now managed a few thousand trouble free miles and I'm taking it to France next Tuesday 3rd Nov.
I would keep an eye on chain tension/lubrication (I find my Izumi chain and sprocket kit I got is awful for road grit etc collecting crap in general so I have to clean it a lot)
Also tyre pressures/wear even though I bought it with the same pair of tyres I'm still using 4k later and they're so no sign of needing a change.
From my research into the bike I can confirm that a common electrical fault around 20kish miles is the regulator to blow (blowing the electrics I think tht basically is) - killing your battery etc and needing a replacement regulator for about 80quid.
But if you say your bike has 9k miles on it you shouldn't have to worry about tht for a while.
Oil change about 5k miles I think the manual says and if you keep it well maintained I hear they last for miles. I'm gonna see how many miles I can get out of mine!

How is it with a screen? I find it can get pretty choppy on the motorway (so I'm thinking of purchasing one).

Only problem I've had with the bike so far other than normal items wearing out is a speedo cable dying on me but it was only a tenna for a replacement.

Enjoy it!

Charlie