IMO, Timur would be only successful of invading North China with the help of Mongols and Central Asians.
South China is out of question, because of sheer size and I don't think Timur can live such long time to complete conquest.
What impact it will bring. Timur will go back, after his dead Timurid Dynasty will be gone in short term.
China is devastated. There will be many wars between Mongols and ITTL Southern Ming and most likely after some time Chinese will eventually re-invade Northern China. How farther they will push is matter of debate. More interesting thing is Southern Ming might become more open to trade, so they might adopt more quickly Western weapons.
Timur invasion of China will bring some stability in Mongols because with the help of Timur, Olziitumur (Buyanshir) Khaan will not assassinated and dominate Mongolia. Northern China will be invaded and devastated and Southern Ming is weakened and not threat for a while. Some time after death of Timur, Mongol Khaans will be totally independent. Depending on luck and competence of leaders, they will be Masters of Mongolian Plain, Manchuria and Northern China till 1500. Culturally they might become more influenced by Timur and Persian culture. Mongols even might adopt Islam instead of Buddhism.
Then as I mentioned Mongols will be pushed back by Chinese. With luck they might keep Manchuria. If they keep Manchuria maybe this will affect rise of Qing Dynasty.