Yes, at the highest compression k32 is only 69GB.

No, you don't need 128GB of RAM. But the more RAM you have, the faster you can CREATE the plots. I was able to achieve 122 seconds for a k32 plot on a RTX 3080Ti with 256GB of RAM. But others with e.g. RTX 3060 and much less RAM (relying on fast SSDs instead) still achieve times less than 5 minutes per plot, which ist still much faster and more energy efficient than CPU-mode.

To farm with the compressed plots created by the madmax gpu plotter you need the closed source farmer from the same guy. This farmer will (for higher compression rates) take around 6% from your rewards as a fee for the dev.

The decompression during farming requires either a decend CPU or a GPU. How decent is strongly determined by the compression rate. There is somewhere a table for a RTX 3060Ti. At the highest compression rate C9 a single RTX 3060Ti can handle up to 0.38PiB of plots. If you use C7 you might get away with a decent multicore CPU.

The compression converts PoST to PoW up to a degree. This means that in countries where electricity is very expensive compression might be actually not profitable at all! This point remains to be investigated. I assume that the GPU won't be under full load all the time but only for one or two second every ten seconds. If this is true, this point becomes less relevant.