SR20DET - The Best Choice
 

I know several people who have put the SR20DET into their 240SX and they are very happy with it. A good friend of mine gets high 20s mpg in town and over 30 on the highway, and he drives it with some enthusiasm. I put an RB20DET in my FB, which is quite a bit longer motor, and I get the same fuel mileage. Assuming you leave the boost levels stock and don't drive like an idiot,these Nissan turbo motors are very reliable.

My preference is to leave the mechanical stuff stock whenever possible and modify the structure. For the RB20 we cut the center out of the crossmember, installed the motor and then just built around the pan with some bracing for the crossmember. Before you ask, yes, I am an engineer, but the modified crossmember was engineered using the TLAR method (That Looks About Right). The photo shows how we did it - don't have a complete picture of it.

Modifying the pan only gets complicated if you have to move the oil pickup. Otherwise it is really pretty simple.

I second the F22c or F20c unless you plan to stay NA. A K20 or K24 would be better for that.