You're thinking of the v-tec-E motor found in the HX and some of the other over-seas models....it would be OBD2 I believe and wouldn't be lucrative for your situation. I would say a d16z6 would burn MORE gas than a non vtec motor but ONLY while in v-tec. At normall driving RPM's the motors perform identically except the DX motors tend to hold torque in the uper rev's becuase of the the non exsistent v-tec function.....which consumes more fuel. If you want that little extra kick in the pants go d16z6. Allthough, if your car isn't equiped with vtec allready, you will need to wire for it. Either isn't a bad choice, just more work with z6.
All of your other questions about wiring and ecu info you can find in the H.S. FAQ referance.
If you keep the D16 out of the VTEC range you should get sweet gas mileage. I have a del so si (D16z stock with a short ram), I generally drive in the 3000 rpm range, and I get right around 38 mpg. Del Sol is ~ 500 lbs heavier, so you should get even better mileage.
On the other note, if I ever get a pizza smushed up against the side of teh box because you're leadfooting it around the corners, be warned...
John