The basics of index shifting are the shifters pull a certain amount of cable and the rear derailleurs have actuation ratio. For older Shimano index rear derailleurs, that ratio is 1.7. The derailleur moves over 1.7mm for every 1mm of cable the shifter pulls.
7 speed shifters pull more cable than 8 speed, which pulls more than 9 speed. In a sense, the rear derailleur just reacts to what the shifter does.
As far as what you should get, keep in mind that in the real world older Shimano 8 speed STI levers from the 90’s are hard to find, super expensive, and tend to be gummed up by the old grease.
Research new shifters you can actually get and pick one based on the reviews; either 3x8 Shimano Claris or 3x8 Microshift R8.
John