B3 is a very good engine to start with and 45k for the overhauling seems about right.
Lets say that you opt for the converson to 1KZ-T here is an estimate.
Engine + Gearbox=125k min
All of this and your fuel average will be almost the same, your vehicle will lose originality and price might not appreciate rather it might endup having a lower market value.
Only the pick will increase as torque of B3 is nearly the same as that of 1KZ-T
So my friend if you are a great fan of 1KZ-T sell your BJ-70 and put extra money to buy an early 1990's model 3 door prado.