Look on the tyre websites – black circles, my-tyres etc. to get basic prices. Then ring around and get quotes for ones that are in your price range. I find that local independents generally beat the websites without being asked.
No, part worns aren't a great idea – tyres can be aged even if the tread's good, and the cost of fitting is quite high compared to the cost of a tyre these days.
Personally, I stick to names I've heard of and had good tyres from in the past – Uniroyal, Goodyear, Continental (although I find eco-contacts horrible).
However, tyres are an integral part of the suspension tuning, and so some that work really well on one model won't work well on another. So maybe go and hunt out a Megane forum for specific tyre advice.
Oh, and I'm guessing from your posting name that you may speak German – one of the big online tyre retailers links to ADAC and other german language review sites (there's surpringly little of this kind of stuff in English), so that may give you a good idea of what's good and what's not…