4203 is very heavy, enormous torque, low top revs - you *will*
need an overdrive or higher ratio diffs.

Make sure you don't get one made for the Spanish market.
Has an "E" in the engine number, e.g. "203 E 123456" -
parts were unavailable when I had to ditch mine four or
five years ago.

The Series II gearbox is somewhat sturdier than the III -
I have one in my III. Watch out for broken bits as a result
of all that torque - I always used to pull away in 2nd - 1st
was a waste of time except on steep uphills.