Fan noise expectations for 2019 27" iMacs
From your experience with previous models, does loud fan noise due to
overheating also occur for the base models, or is this a problem of
the higher end processor choices?

Unfortunately, in the Apple community, this is a common miscorrelation.  So, let's first set the record straight...
Fan Activity ≠ Overheating
Just as the radiator fans in your car periodically come on to move additional air across the radiator, the fans in your computer (PCs and Macs) spin up to move additional air across the fins on the heat sink in an effort to dissipate heat.
Hearing fan activity is a perfectly normal and expected behavior regardless of the processor, memory, or general specifications of the machine.
Fan Noise
There are only two fan noises that you should be concerned with:

Squealing and/or grinding indicating a failed bearing
No fan spin at all indicating a full failure

Overheating
Like your car, you will know exactly when your computer overheats.  It will slow down and eventually shut off.  Granted, there's much more fan fare with a car, usually with steam and coolant gushing everywhere though with the computer, lost work can make you feel pretty steamed and some choice expletives are also probably going to come gushing out at well.
However, unlike your car, this is by design.  Your processor will thermally throttle it's speed limiting what it can do and if all else fails, it will power off in a bid to save itself.
It's not just the CPU
The fan spins up because the overall temperature within the chassis is high enough to warrant it.  This can be from the CPU, the GPU, the drives, the logic board and in the case of AiOs (iMac), the display and the power supply.  Even the ambient external temperature is a factor.  All of that contributes to the internal thermal conditions which may necessitate the need to spin up the fan.
TDP or Thermal Design Power
TDP is the maximum amount of heat that your processor will generate on average use.  A higher number means it will allow more and higher processes before it gets into the &quot;red zone&quot;.  In other words, a processor with a higher TDP will do more before the fans spin up or throttles back to cool off.  In your case, comparing the 9th generation i5s with previous generations, you get a higher TDP because the chips are more efficient, not that they generate more heat.
Fan Control Software
Unless you intend to manually increase the fan speed in an effort to keep things extra cool, this is the worst thing you can do.  Slowing the fan in a bid to make it more audibly aesthetically pleasing will only result in one thing - putting a cap on the processing power of the machine you paid money for.  This is like buying a high performance car and immediately installing a restrictor plate.
Bottom Line
Buy the iMac that meets your needs (CPU, GPU, memory, storage).  Fan spin shouldn't be part of that equation.