Vehicle wont turn over than back fired

Bought a 04 chrysler pacifica 3.5 liter awd for my
wife at an auction.  What a nightmare it didnt make
it 2 blocks and it knocked really loud than shut off.  
It then would start afterwards but would knock
than lose power.  Hired a mechanic who suggested
replacing the 04 motor with a 05 motor he located
at a pik apart. After he installed the pik apart motor
he was stuck on figuring out why it wouldnt start.  
2 weeks passed And no word from him so I hired
another guy tow it to his place to work on it and he
also cant get it going.  It cranks than backfires?  In
need of help limited on funds and worried im S. O.
L.