High Engine Temperature Guage and A/C shutting off

2000 Ford Ranger, 4.0L, 6 cyl, 5-spd manual transmission, 95K miles

Haynes refers to two engine coolant senders, one that drives the guage
on the dash (the sending unit) and one that communicates to the engine
computer (the sensor). Advance Auto Parts refers to a "rectangular"
and a "round" ECT, and sold me the round ECT (part #TSU83, Sensor) as
there was an A/C notation in the computer. When I got to the
engine/cylinder heads, I couldn't find a matching part to this
"TSU83"....Even the electrical connections don't seem the same.
Question #1) Is this the right part?
Question #2) Does anyone have a better picture than Haynes (p3-5) on
where the sending unit is located?
Question #3) Barring that, could someone walk me through a description
of how to find it?
Question #4) Should I change out my radiator fluid and
thermostat/gasket as well?
Question #5) Any problems with using the orange coolant fluid....or
even the Prestone fluid that claims to be useable with any color fluid.

Thanks,
Calvin