i dont know the specifics on the smaller diesels but heres something to think about. if you get a diesel with mechanical injection there is no computer to worry about, very little wiring to take care of (1 wire run to the injection pump shutoff solenoid, wires to the lift pump located along the fuel line, and wires to the starter). if you have an old carb'd truck as your starting platform there are no extra wires you need to run. simply run the wire that went to the distributor to the injection pump shutoff solenoid, the starter wires to the starter, and the fuel pump wires to the lift pump (some people use the original fuel pump as the lift pump).

if you go electronic, well ask mark (willy mammoth) or check out his build thread back around page 90.

some mechanical injection motors to think about are:
-nissan sd22: 2.2L that is not powerfull or quick, but amazing on fuel economy. fairly easy to find as well, but will require a new bellhousing. used in some generators and 2wd nissan pickups from the 80's.

-nissan sd25: 2.5L turbo diesel that is more powerfull with slightly less fuel economy, also requires a new bellhousing. little more difficult to find but is a good option to go with if money is tight. used in the UD trucks and a few other places.

-cummins 4bt: 3.9L turbo diesel that is easily the most powerfull of the 4cyl diesel motors. frame reinforcement, heavier duty front springs, and upgrading your t-case to a 23 spline would be required because this motor is easily tuned to over 400 ft/lbs of torque. weight is similar to a gm 6.2L diesel (roughly 800 pounds dry). great on fuel economy though and easily the strongest 4cyl available. can find this motor in construction equipment and box trucks (UPS trucks for example). baseline for this motor is around 240 ft/lbs of torque.

none of the motors i listed are a direct bolt in, afraid i dont know anything about the toyota diesels.