I just splurged for a Mr. Aqua 6g bookshelf tank. Love it! They may not be low iron glass, but for the price they are pretty cool. Aquatop also makes some nice cube and rectangle tanks at a decent price, under the Tru aqua name. Like so:
On a tank this size, you have a lot of filter choices. Aquaclear HOB filter, a 20, is very flexible on media and easy to clean and service. You might need a sponge over the intake, but that is easy. Or, you can go canister with something like an Eheim 2211 or 2213, turned all the way down. Tons of filtration, and little more labor intense to maintain, and you don't have a HOB messing up the pretty tank view. Or, you could try a Zoomed 501. It is a turtle filter that does well for nano tanks. It is supposed to sit next to or behind the tank, but some folks have no problem with it underneath. I would research that option before you do it.
For a lid, I would get a piece of plexiglass or glass cut to size, and install some rubber feet or clips on it. Ehiem makes some for the Aquastyle that can be ordered separately and they work OK.
For a heater, I would go with a Hydor Theo 25 or 50w adjustable. Definitely need an adjustable.
Light is where you have about a billion choices. The big thing is you want something that will last, and not heat up the tank too much. You also don't say whether you have live plants. I am going to assume you do, because otherwise it matters very little what you use as long as you keep an eye on the tempertaure. It can be as simple as a desk light or two with CFL bulbs. You can get LED clamp lights from Finnex, Depp Blue, and a host of others. There are T5 fixtures, build your own LEDs, the new Current Freshwater+ which has all kinds of cool effects on it (thunderstorm, cloudy day, sunny day, moonlights, etc.). If you have plants, do a LOT of research on your light before purchase. It is all about PAR numbers now. You will need about 30 at the substrate. It can be higher of course, but at some point you will need cO2 and a heavy fertilizer regime.
Hope this helps! If you just want a simple light, low light or no live plants, take a look at Marineland doublebrights.