For such a small tank, I advise not to get the live rocks and keep the power filter. The live rocks would take too much space from an already small tank.
Be sure to have enough media in the filter for the biological filtering. Carbon is nice to have, but not necessary. You can just use a simple circulation pump if you feel like the water is not moving enough.
Also buy your own test kit. Its a basic must-have in this hobby. I never trust anyone at the LFS
If you don't have a HOB
skimmer or refugium (growing macroalgae), then definitely keep up with the 50% weekly water change. Puffers are messy and those two items are the only way to postpone water changes since they help keep the water cleaner. Poor water quality is often linked to premature puffer death.