What new units, disasters, techs and wonders do conquest have? And is it worth to buy it, if its worth to buy it can it been downloaded and regersted online like i did with civ3 gold?

You can find out abou the new units wonders etc on the official civ3 website civ3.com and in several threads on this site as well.
You can download conquests and try free for 3 hours then have to register and pay if you want it.