In theory, there's no specific amount required to launch a network attack.  You only need at least 50% to sustain the attack indefinitely, although, as you rightly pointed out, initiating such an attack would likely lead to a significant drop in price (unless the attacker exclusively mines empty blocks, good luck moving coins in that case, lol).

The pressing question is: Why would China or any other nation undertake such an action?  What would they stand to gain from it?  Investing in and implementing new tax regulations could offer them benefits, providing additional revenue sources for their tax systems.