Can a old line set be flushed and reused to replace a home hvac system

Not really, if you are replacing an old r22 system with another r22 unit, after flushing you can
But replacing r22 unit with condenser using a different refrigerant like 410A is a no-no
Doubtful you could get every drop of r22 oil out of lineset

Yes they sell a flush kit to go from r-22 to 410-a. You must do this correct because the two do not mix and can damage your new unit