Diana's advice on fertilizers is good.
That said, I tend to use API leaf zone for the potassium + iron. I can get much cheaper than either Seachem potassium or Seachem Iron, much less both together. I use it in conjunction with flourish comprehensive and API CO2 booster (similar to excel).
I would strongly advise against skipping water changes, particularly in a beginner tank. Yes, it can be done with just the right fish-to-plant ratio. However, this is really takes a lot of experience to get right and have it work long-term. Too many fish vs plants and your nitrates will climb and eventually kill your fish. Too few fish vs plants, and there won't be enough nitrates to keep the plants growing well, and it will devolve to an algae farm.
Water changes serve two purposes:
1) Removing nitrates. Yes, plants can do this as well, as long as your plant to fish ratio is just right, as noted above.
2) replenishing calcium and magnesium. Yes, you can add this with more fertilizers, or by adding a bit of crushed coral, etc... but water is cheap.
Collectively, this saves you worrying (much) about a pair of nutrients, and saves you from having to balance your stock ratios closely.