engine problem

hi i hope this is correct place for questions. i bought ford mondeo 2.5 v6

1996 few weeks ago. when i got it home it developed a top end rattle so i bought engine out of scrapyard for £200 put this one in and had same problem. i am now stuck as they wont take engine back or replace it so im back to square one/ as im not mechanic i would like to know if anyone has had this prob as i think it is hydrolic tappits but not sure. i thought ford parts were cheap but was soon proved wrong. im now thinking about replacement tappits is this easy job for DIY enthuisiest or not as im worried about the timing on the camshafts. please help