2004 Ford Explorer - Need Advice

Bought a 2004 Ford Explorer and had a problem the first day. Had to replace the transmission. Ran about $3,700. Had many mechanics say that the motor was in great shape.

Had a problem with leaking/some noises. Took it to a mechanic who said it would be an easy fix. 3 trips there - had the car there for about 9 days in total. This ran about $400 but we're not convinced anything was fixed.

Took a trip down to NC a few days ago and the car started trembling and eventually came to a stop. Left it at a shop. Mechanic said the problem is a broken valve spring. (Would cost around 1,400). Then he informed us today that there is internal damage to the motor which would cost about $5,900 +.

Does anyone know if this is a common problem with this car?
And what other problems are common?

Would it be worth it to have it towed back home (132 miles away) and buy a new motor and have the springs fixed? Or should we let them keep it and junk it? (;

Thanks in advance!

What engine 4.0/4.6? Is this a sport trac? 4x4?

Xls 4dr suv - 4.0L