It is a 2 hour job or less on a 84 GTI. about as much time burping and
doing the R&R.

So what does defunct mean? I have had them start leaking at the seams
but never clog. I do have one on my 22 year old truck that needs to be
cleaned. I have replaced water pumps that have failed on a few cars.
Some water pumps use plastic impellers that get loose and do not pump
so swell. There are also different impeller sizes. If you install one
that is too big you have a problem. One that is too small and it will
not pump properly.

It has been a while but I think you can leave the fan attached to the
radiator. I would go one step further and replace the water thermostat
by the water pump.

Note that there is a burping process for these cars. I fill up the
radiator any way I can. I dump antifreeze in the top of the radiator
if it has a top. I dump it in the long top hose. I think in 84 they
have a coolant reservoir which I leave the top off of until it has
finished burping. I leave the heater control to full hot inside the
car. Important to know that the radiator can take up to another quart,
more or less of antifreeze after you fill it. There is a big air
pocket somewhere that takes a bit of time to get free.

If you have a factory turbo diesel it may not burp the same way as gas
engine. The gas engine makes plenty of heat on its own that burps if
there is fluid flowing around the thermostat by the water pump. Once
that thermostat opens up the air works its way out. My 2003 diesel
does not make a lot of heat at idle but it burped out ok. Note that
the 2003 engine and cooling system while similar to a 84 is not quite
the same.