If you already have an air pump I totally recommend going the route of a sponge filter. Ths is a very good one, I use the smaller version in my 10 gallon. [ame=http://www.amazon.com/Hagen-A902-Double-Sponge-Filter/dp/B0002AQH00/ref=sr_1_4?ie=UTF8&qid=1369793280&sr=8-4&keywords=hagen+elite+sponge]Amazon.com: Elite Double Sponge Filter: Pet Supplies[/ame]
However I'm sure there are cheaper options around, I just love how this one works :) Just search 'sponge filter' on Amazon or eBay and you'll find a ton of options.
You don't HAVE to go with sand, I just love it so much I unconsciously push it on people. A cheap sand option is either Quikcrete Premium Playsand, or Black Diamon Blasting Sand. Both need rinsed, or just lots of water changes as they do have 'dust' on them. You are totally fine sticking with gravel if you want to, though.
You can always ask around for plant clippings in the classified section, many members are willing to send freedbies or cheap packages :) I just sent off a bunch of plants or I'd send you some :( I may have an extra amazon sword and some other things in a few weeks, though.
You can check walmart for 6500k/6700k fluorescent bulbs, just make sure they're the right length and are T8s (which is what I'm assuming you have? It's usually the standard.)
Root tabs can be found on Amazon, there are many different brands to choose from. Last thing would be Seachem Flourish Comprehensive Plant Supplement. Oh, and the test kit.
I know it sounds like a lot, but if you're not in any rush you can buy them one piece at a time, and all of these items will last quite a while :)
It's up to you, but personally I get tons of enjoyment setting up planted tanks and watching it grow and change :) Plants also help keep the water cleaner for fish, and they really seem to enjoy them :)