"and flush the system well with a R12 charge..."

Really bad idea... but as soon as you start studying for your test you will figure that out...

Both oil and solvent flushes are available... which I think is good due to the viscosity... to take out particles which might need to leave your system.
Then , if you want to, use Nitrogen to flush the solvent out...
All the latter are both cheaper, better, AND LEGAL to do compared to what you mentioned first....