Unless you own many bikes where you'll be doing lots of cable replacements, you're best off buying one of the kits, which include 2 inner wires, enough housing for both brakes or derailleurs, and ferrules and crimps. You'll need 2 kits, one for brakes and one for derailleurs. There are a number of kits of decent and better quality offered by Jagwire and others. At the very least look for die-drawn stainless steel inner wires with lined housing. The stainless option is less important for brake cables where quality die-drawn galvanized wires cost less and work fine.

You should also by a cable cutter which can cut the inner wires and housings without crushing and fraying them. That, an emory board, file or grinder and a bit of thin oil, along with a bit of hand skill is all you'll need.

If you're considering buying bulk to save, note that brake and derailleur housing is different and neither is usable for both, plus ferrules and crimps come in large bottles so it'll be a long while before bulk saves you anything unless you have a number of bikes to work on.