We have a ZETEC motor.  Cheap and very reliable.  Plenty of power.  No transmission changes required and when I get horsepower hungry I can add the turbo.  Just need to get used to running it at 5K up hills.  We are at 5500 feet and the power is still strong.  When we are on the flats and at a lower elevation there is no problem running at 80 MPH.  Reliability is a biggie.  Was great running in Baja MX last year and not worrying about the motor and breaking down. Goes a long way.  Motor only cost about $500.  Kit total was around 6K.  Really good support and documentation.  I�m sure that are other good ones out that but this one works great for us.  I want to be able to get parts anywhere and since I put it in I understand it. 
Mike Saint-Amour