Went to buy a 2014 outback 4cyl with 73,000 miles, my question is it normal for the engine to knock for the first couple of minutes?

Some of the sti ej207 have a little bit of piston slap for the first few mins after cold start but that's due to the factory forged pistons. 

V3 /v4 do sometimes have a bit of piston slap when cold ,due to a different material used but generally run fine .

No engine should have knock though and if they do ,its normally a sign of early crank bearing failure