Cleaned up the exterior and doing the normal new seals etc. to install in series III car. Is it a good idea to flush out the inside? What would you use?

 

 

Don's iPhone using Classic Zcar Club mobile

You'll stir up the debris that would normally just sit on the bottom of the case, and disperse it through the various bearing surfaces.  Flushing the transmission is not specified by any manufacturers as a maintenance procedure.