I firmly believe that it's a good idea to learn pawn structures before getting into opening theory in any depth. You probably have a decent idea of what openings you are interested in, so you should study the relevant chapters in Soltis. After that, I highly recommend Chess Structures: A Grandmaster Guide. Some structures are covered in both books, but the authors' approaches are quite different and you can learn something from each.
One caution: a few basic structures should be studied even if they aren't classified under your chosen openings. For example, the IQP can arise from almost any opening, and knowledge of how to play it (and its frequent descendants, the isolated pawn couple and hanging pawns) is essential chess knowledge. Ditto with what Soltis and Flores both call the Slav formation and the Caro formation.
I used to be an advocate of Pawn Power in Chess and still believe it is a useful book. However, it doesn't cover much of anything that you can't get from the pawn structure books and either Pachman's Complete Chess Strategy or Marović's four volume work (Understanding Pawn Play in Chess, Dynamic Pawn Play in Chess, Secrets of Positional Chess, and Secrets of Chess Transformations).
Should I build a repertoire (currently my only knowledge of opening lines is very general, have not studied any opening books) then study the relevant chapters in Pawn Structure Chess or study the book then build a repertoire later with the knowledge gained?
I want to increase my performance but I prefer to do it in the order that will help me improve more over the long run.
If you think I should first build a repertoire I wouldn't mind some suggestions. I don't have a particular style because I suck but I want to build a fighting repertoire. When I "grow" into the repertoire, so to speak, I want to fight with both colors as if its always the last round and a win is needed.
Also, if I build the repertoire first then complete the relevant chapters of Pawn Structure Chess, should I finish that book or move on to Pawn Power or Chess Structures: a Grandmaster Guide and come back to it later?