not to mention the age of tyres has a dramatic effect on grip and ability to handle heat at high speed usually resulting in blowouts.
there was a documentry not so long ago about the dangers of part worns due to various hazzards that can effect the tyres performance.
the extra tread a new tyre has also has a lot to do with how well the tyre can disperse water to prevent aquaplaning, therefore it stands to reason the less tread you have the more incapable the Tyre is at dispersing water and more likely to aquaplane which is why most european countries have a 3mm limit not 1.6mm.
for the money new tyres with usually double the tread is far better economy and reliability and safteys sake.
i was a tyre fitter nearly 10 years ago and we used to get tyres in from europe who had a 3mm tread depth limit so most of the tyres we sold as part worns had 3-4mm of tread. ok you have some good name brands that may be ok after a few years but i would definately never trust a cheaper second hand budget tyre even if it were a tenner.
its been said before but the only thing between you and the road is your tyres, so the FIRST performance mod you should ever do on ANY car is to fit good quality tyres and pay the premium because you will need it sooner or later.
I dont mind project cars but I HATE SANDING!!!