The '68 was the first year C3 and as Rosco points out, did have some build quality issues. Although physically little different from '70 to '72 cars, annoyingly for later owners it featured a multitude of one year only parts. The '69 t0 '72 'chrome bumper' cars do command higher prices than the later ones - they're more powerful, they're arguably better looking and mainly because of the three year manufacturing window compared with the nine years of the later cars, they're rarer.
Three years ago I bought - in the UK - a recently imported '71 convertible, a base 350/270 (350 cu.ins or 5.7 litres and 270 HP).
It was a 44000 mile car that had been undersealed from new and was essentially a 'survivor', meaning that it had been maintained enough to keep it on the road but had been largely untouched by 'restorers' of questionable ability. Underseal is always a mixed blessing but in the case of my car the chassis and birdcage were in good condition. It had the (to me) desirable features of manual transmission and power brakes and steering - not all C3's have these. In addition the condition of the mechanics was commensurate with the mileage, which given the condition of the interior (my upholstery is original), seemed believable. The car had been resprayed at some time in original colour but to improve it now beyond a 'ten foot car' it needs doing again.
The car cost me £25K and to date I've spent around a further £10k on replacement parts (all labour has been my own). I'd be very surprised if that investment would be recouped were I to sell the car now, but that wasn't the point of acquiring it. It is becoming an increasing pleasure to drive as 'end of life' parts are replaced and 'niggles' sorted and being retired working on the car gives me pleasure and keeps me out of mischief.
Get clear in your mind the features of the C3 that are important to you and how much you are prepared to spend, both in time and money, to end up with the car you're going to be happy with. Do you have the equipment and the competence to carry out work on the car or are you going to need garage services? If I'd needed outside labour to carry out the work I've done on my car, the £10k parts bill would have been at least doubled.
The C3 is an iconic and striking car and it's easy to make a purchase with your heart rather than your head - there have been tales on this forum of purchases soon regretted as the true state of the acquisition became apparent. Make an honest assessment of a prospective purchase and be realistic about the work that any 50 year old car will need. As always if a car is cheap, it's cheap for a reason. Get it right and you'll have a car that attracts attention wherever it goes, that you'll never get tired of looking at, and will be a blast to drive. Good luck!