First my background:
I had a whole lot of C's (as well as A's) in the bike hire business and still have one which I ride privately - 110,000km on the clock after a rebuild at 100,000km
I work "a little bit" on bikes here in Cape Town. Mostly DS bikes.

The most important thing: The "C" is IMHO the one and only KLR you want to take. It neither has the early A models pathetic front brake or its toothpick fork tubes nor does it share the soggy & squishy suspension which spoil both the A model and the current KL650E's. The "C" feels like a different bike and does not seem to have anything in common with the other KLR's ecxcept the engine.

If you like your C models and you buy either an A model or a current KL650E you will be in for a huge disappointment.
The one and only flaw you may find on your bikes is the relatively small tank (14l). This is not really a show stopper since you can fit the IMS 6.6 gal plastic tank but you will have to modify the seat (remove cover & foam, cut and plastic weld seat pan, re-upholster).
As for some comments above: There is NOTHING which makes the A or E model rear subframe superior to the one of the C model. They are near identical. The C's beach towel holder actually has much more sturdy attachment points than the sturdy looking but pathetically flimsy carrier rack of the other models. The C does not need the rear brake bracket which is an essential mod for both the A and E. The C got the much better KDX250 gearshift lever in 1995. The A got it in 2007 : Tell me more how great you think the KL650A is.

If you like your C and decide to buy overseas / in the US of A rather get Suzuki DR650SE's (I also have two and over the years had eight in the bike hire) Forget the current KLR you will need a LONG list of mods and extras from crash bars to protect the brittle bodywork to extensive suspension upgrades.

A rebuild 60,000km KLR engine will be good for many more kms, check the bikes thoroughly and you've got little to worry.
The C's rear shock albeit simple is strong and lasts well as long as you don't overload the bike - which is much more difficult than with the other KLR's - the shock spring is 30% stiffer.

Re. oil burning:
All KLR's seem to burn oil as soon as you take them over 120km/h. I had customers which managed to burn almost a litre of oil on 1000km riding my private KLR two up and probably balls to the wall. Since the engine was rebuild only 4,000km earlier I was mildly concerned. Kept an eye on it over the next 2,000km (whilst I rode it "normally" ) and had an oil consumption of maybe 200ml

The 2008ff KLR oil burner issue is due to poor (out of round) machining of the cylinders. I've rebuild a whole lot of those engines and have documented the story on the wild dog forum (with pics). The poor machining of the cylinders is visible to the naked eye (creates hotspots after a few thousand kilometres). Wear of the side rails and ultimately compression rings and pistons is a secondary effect caused by the poor quality cylinders. A good quality rebore obviously fixes this problem.
According to my experience even a hard done by KLR in the bike hire did 75,000 - 80,000km and more before requiring a rebore. A well looked after privately used one will probably do as much as 100,000km. The bottom end is rock solid as long as you don't kill the big end by overrevving it. Gearbox failures are also rare.