I've only just noticed you snails don't seem to survive.
Try MT snails, usually if your LFS if they have any(and usually they do, cause they clog thier filters up)
You can get them free, I've done it
ive also gotten them for free at the lfs. they happily picked out a bunch (i love the snails in my tank. i suggest everyone have snails..they are a good indicator of a lot of things).
in a small tank such as a 3 gal, snails are the best option. apples snails may get a bit too large after a while., but im sure you could find someone to take them at that stage (which could take a long time depending on amount of available food(ie not overfeeding the tank).
and i also agree that you dont need to buy everything at once. 2 of my tanks dont even have a hood.
the oily film you have noticed is likely from the natural oils in the fishfood whether pellets, flakes or freezedried. a filter disturbs the water and mixes it in, or polishing pads in a filter would likely remove it as well. also from hands in the tank if you dont wash them beforehand. there is more oil on our hands than one might think. always a good idea to wash hands before and after going into the tank.
to the OP-get the biggest tank you can afford and have room for. you can even make a simple filter yourself for whichever size you choose. very easy and they work much better than small commercial filters. if youd like advice on how to, id be happy to pass some instructions along. i use them in all my tanks.
cheers.