I had two, a C2 Model and Later an E1 (They never made any D's and the last of them were F's)
The only "known" fault I know of is occasional gear selector problems, cheap enough in parts and would be a simple enough job...... if it did not require the engine on the floor.
I think these problems are quite isolated, and typically happen at higher mileages (on a sportsbike I think that means over 30k) I think the symptoms are dropping out of 5th gear (or higher gears)
Check in on a ZX9R forum would be the best bet for details of what to look out for.
Apart from that they are great bikes, even my earlier C Model was brilliant on the road (but I did not like it on the track) and the E Model even better, much sharper looking.
Due to never quite being as good as the latest Blade or Gixer means they are available for ridiculously low money, and for that you get a damn fast, great handling and reliable bike.
I assume your not riding around the world and unless planning to track the thing the handling is fine on the road, a shade hard at the back end but these ain't built for back lanes.
They ride surprisingly well two-up (Probably due to stiff rear spring) and I took mine away for a long weekend with the Mrs + Luggage and it was pretty damn good.
I would personally go for an E or an F if buying again, if only for styling, but I think they did improve power and handling a smidge with each update.
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