My mistake with regards the engine - I was thinking of the GPZ250 - and engine which was perfect as a 250, but overstressed as a 305cc, which is what happened when learner laws changed and made the GPZ250 obsolete overnight. As a 305, there are a number of problems.
Firstly - the cam is in the head, and relies on good oil flow. This is not good, because (1) the oil pickup pipe is small and restrictive, (2) it's fitted with a gauze filter, which is prone to block, and (3) is hidden behind the oil pump which is (4) driven by a nylon sprocked that is prone to stripping the teeth. (5) The camchain tensioner is also a weak point.
If you're going to buy a 305, then be aware of this. You want to know what the TRUE mileage is - in the UK, we can now check this through the government DVLA website. Any vehicle over 3 years old has to have a yearly safety check, and the mileage is recorded. When I went to buy my bike I queried the mileage, then proved the bike had significantly more miles on it. I also pulled them up about the uneven brake levels from the rear brake.
You want to be asking when the valve clearances were checked - and check them yourself the first chance you get anyway. You want to see receipts to prove the oil and filters have been changed regularly - and I mean ideally new oil every 1000 to 1500 miles, and a new filter every 2-3000. You want to make sure (again, first chance you get) that the nylon sprocket driving the oil pump is in good sound condition, with no signs of any cracks or bad wear on the teeth. Then you want to dig further in and make sure that the oil feed pipe behind the oil pump has a clean gauze filter. You need to do this every time you change the oil. Do this, and you'll have a better chance of a reliable GPZ305. Don't take my word for it - there's a german website -
http://www.gpz-305.de/English/index_english.html - that will give you some great information.
This PDF gives a bit of useful information on the bike as well:
http://www.z250.co.uk/CMM%2001%2006.pdf, although it doesn't go into the significant reliability issues far enough as far as I'm concerned. This page shows the experience of some riders/owners of the GPZ305:
http://yewemmgee.blogspot.co.uk/2012/09/kawasaki-gpz305.html
Given all this, I still bought one - the green thing that is my avatar It's like anything else - if you know about the weak points, you keep an eye on them - preventative maintenance is the thing (and possibly some common sense modifications).