I did the same when I was first starting and have used Imagitarium water with good success. The 5 gallon size is perfect for a weekly or biweekly water change and there is minimal evaporation with the stock lid (just know where your water level is and top off with a little RO water every day or two - a gallon of RO water will last you a month or more).
You should be ok with what you have to add a couple of clownfish (they’re pretty resilient). If you want to test to be sure, buy some ammonium chloride at the hardware store (be sure you’re getting pure ammonium chloride) and add enough to get your ammonium level detectable - if it is back to zero in 24 hours, you’re cycled enough to add a couple of clowns. Let them be alone, preferably with lights off (to avoid major algae issue) for a couple of months and you will build up your bacterial population. You can then begin to slowly ramp up your lights, add a fish or two, and cleanup crew as the algae starts to grow. Can also start testing nitrates once a week or so and once they start reading 10-20, that would be a good time to get in the habit of a weekly 5 gallon water change/tank vacuum. After that, you’ll be well on your way. Slow and steady. Good luck!
