Use tap water if at all possible. Water changes are the key element to a healthy tank and having to make trips to get water is gonna get old pretty fast.
One thing you need to purchase asap is a master drop test kit. You will need this when cycling, which as mentioned above is something you need to start doing NOW, not later. If this is your first attempt at a tank then doing a fishless cycle is the way to go - much less stressful for you since you don't have to worry about killing your fish in the process, and also a good time to learn how to use your test kit and how the cycle itself works. A typical cycle from scratch (meaning you have not seeded the tank with bacteria from an existing filter) typically takes about 6-8 weeks. If you're going to use Prime then I would actually recommend getting an ammonia test from Seachem as the other ammonia test kits can give you somewhat inaccurate readings with Prime. The Seachem tests can be found on Amazon for a decent price.
Another thing you'll want to get is a smaller hospital/quarantine tank. All fish should spend a good 3-4 weeks in quarantine before adding them to your main tank, because if they have an disease or parasites you can find out and treat before the main tank gets infected/infested. This tank will also need to be cycled.