You can reuse your original guide rod and spring, but for the cost we prefer to use a fresh combination with every build. Springs are wear items with a lifespan that should not be ignored. It's not uncommon to run a spring out past 10,000 rounds, but we have seen issues during competition use with much lower rounds counts. We prefer to adhere to a "better safe than sorry" mentality, especially when it comes to firearms.
To quote GLOCK directly from an instruction manual: "The recoil spring should be strong enough to move the slide forward reliably to chamber cartridges even if the pistol is somewhat dirty, dry, or the ammunition is not perfect. With an UNLOADED pistol, point it 45 degrees upwards and pull the trigger. While holding the trigger back, pull the side to the rear and release it very slowly. The recoil spring should be able to push the slide completely forward and fully into battery. This test verifies that the recoil spring is strong enough to chamber ammunition despite less than ideal circumstances."
One important note about the above test: This is intended to be applied to carry and duty pistols using a factory recoil spring weight. It's common to run a lighter weight recoil spring in a competition/match pistol, especially when using lower powered match ammunition. In these applications, it's quite possible that the slide will not fully return to battery under spring power alone (especially prior to a thorough break-in), however it will still run perfectly during live fire cycles.
You can see our selection of recoil springs & guide rods along with our various other upper parts by clicking here.