might buy 72 supreme 350 ci. wondering what I might expect the repair to cost and might there be unknown longer term engine damage revealed by the lifter tap ?

 

Since Olds engines have non adjustable rockers, lifter noise is a common occurrence for hi mile engines. You may need to replace rockers and perches, lifters, and worst case possibly a cam. The only way to find out the condition is to pull the intake and inspect everything.

Another issue may be low oil pressure and that can be related to bearing clearances which ultimately will lead to a complete rebuild.