Make the sprites as good as you can within a reasonable amount of time. Don't stress too much about it. You can always do a second pass later on. And even if you decide to pay a professional artist, your sprites would still be useful as a vehicle for communicating your vision.
Yes! There are many issues hiding in any sprite art that will only become evident when you see it all together. Don't draw a dozen dudes wearing browm boots only to find they clash with the colors you used on a dozen terrain tiles.
