Couple of things. In peace time buying and getting even a 100, let alone 1000 planes takes time. Much longer time than such a war would last. Perhaps some extra ammuntion/missiles can be bought (even those in smaller quantities, if russia already has them in stock to be sold) but that's it, in war times there wouldn't be any real arms purchases. What you have in the start of war would be all you'd have during it. Also, it's quite possible there'd be instantenous arms embargo if china invades, west still has quite the leverage over russia economically.
Secondly, if US is determined to fully engage china, there's little chance china would have time to transfer a couple of million troops to taiwan. When US starts preparing, it'd probably take 2-4 weeks to organize, transfer forces and achieve air/sea supremacy. During that time chinese transport capacity would be compromised as well.
But even that's somewhat beside the points as US would not invade taiwan in order to free it. What's the point? All they need to do is maintain their control of air/sea. Imagine if suddenly there was an inpenetrable wall around iraq. All that mighty force US has in there would crumble in matter of days/weeks. No support, no reinforcements, no supplies of any kind, fuel/ammo/food. Same thing for chinese trapped on taiwan. Sure, lets say they have a million troops there by the time US comes. It's a million mouths to feed. They could use what they find for some time but ultimately they can not support themselves in a infrastructure demaged hostile country with guerilla attacks on every corner. Maybe they're crazy enough not to surrender and then it'd take weeks if not months till they're overwhelmed...but in the long run they are trapped and they will lose.
Btw, i also have a feeling at least some US generals/politicians would love the opportunity/excuse that is china's invasion of taiwan in order to bring down china's armed forces down to size. They've been getting antsy over last decade or two and it's obvious china is starting to catch up. So, the sooner the war happens, easier it'd be for US to win.