Not all ROCN vessels are bought, a proportion was manufactured in Taiwan. ROCN does have manufacturing capabilities, they are now manufacturing the Kuang Hua VI (missile boat) and built the Cheng Kung class frigates (based upon Oliver Hazard Perry Class). While China has a huge manufacturing capability, it is dependent on outside resources in order to actually manufacture. There would also be economic sanctions (as mentioned in the 2009 Report to Congress - Military Power of the People's Republic of China) and other actions taken that could severly hamper Chinese manufacturing.
Actually, I cant agree with the latter part of what you have said here, chinese naval vessals are NOT dependent on outside resources in order to manufacture, I cant really find many navalsystems that China are currently using on thier naval vessals which they cant produce by themself, they are all either self developed or license produced, including the power plant.
And about sanctions, today we have come to a point that any sanctions by xxx vs china will also mean an equally damaging sanction by china vs xxx, more so for each day that going by, also the fact is that china is proberbly one of the only 3 nations on the planet that can build most of its military equipment all by it self, the other 2 been Russia and USA(and EU if you can consider the entire union as a nation).
Anyway, modern ships cant be build fast enough to replace a intensive modern war losses.