Stevens:
The circuit you have found is commonly built by many and the parts as you see them are for about 4 to 5 dollars plus the small printed circuit board, the case and wiring.
The circuit may have the mods I suggested many months ago to a group that have been discussing and building them.
For one and for the time you will need to search for parts and assembly, if you are capable, my suggestion is that your time has higher value, just buy it and give it a try.
CAVEAT EMPTOR the circuit works and do good desulfating BUT it takes time, sometimes weeks, the device is good to have it connected when charging the battery to do desulfating during charging to extend the battery life.
The circuit, by what the advertisement reads, seems to have a temperature sensor to reduce the current been used by the Power MosFet.
I used to make them commercially for large battery banks, indeed in different and capacity.
The circuit using capacitors and rectifiers, I do not recommend they may be widow makers and when I used to experiment technically with them, they were quite inferior than the electronic one, requiring about 5 to 15 times longer periods to attain the same results with the same type of currents.
Nando