Where to start? Let’s go with the simple stuff first, mmmkay? What car are you working on? Any mods? Are you working on it in your grandma’s basement? All kidding aside we need to know the details on what car you are working with.
Do you have a link to the eBay add for the springs?
Working on my 86 GT ..So far as mods starts small with brake and suspension mods slotted and drilled brake Rotors from Stop/ Tec with Ebc red pads
The lowering springs are the last mod
Spring rates are very similar to Ford Racing “C” springs. You can likely look around for recommendations for those springs as to what dampers will work.
Given the rate, and drop, you’ll likely want a better damper to control the spring as they will hit hard and bottom out with stock replacement type struts and shocks