i bought an eclipse 3 gallon tank. it comes with a light and filter and you can probably find it on sale for about $35.
i've used both aquasafe and bettasafe as a water conditioner. it takes about 7 drops per gallon; not very much, at all.
i agree that the heater is very important. You can get a 25 watt for that size tank, though I also use a 50 watt in one of my 3 gal tanks and it works just fine and will be enough in case i have room to upgrade tank size in the future.