I would use something like a credit card to scrape the growth/residue off. Your local fish store should sell plastic blades if you don't have a credit card that you are willing to damage. Be carefull not to use anything too abrasive or you'll end up with tons of scratches. Definitely DO NOT use anything that is metal. A metal blade will damage your tank. I would also avoid using chemicals or any other solution to clean the tank.
In regards to the filtration, you have a couple options. The most common for that setup is to use a wet/dry filter. You can find an old used tank on craigslist and make one yourself pretty easily. I would recommend buying the biggest size that you can fit in your stand. Buying one new is a complete ripoff. Just search in the DIY forum and you'll find tons of good ideas. The premise of an overflow wet/dry filter is that water overflows down through the larger hole in the overflow into a wet/dry sump (through filter floss, then through biological media) and a pump placed in the sump pushes water back up into the main tank through the smaller return hole (the one with the barbed fitting in your photo).
Option #2 is to use a canister filter. You can have the overflow go through a canister filter (like an FX5). I would personally go with the Wet/Dry. You have more options (you can add a lot more media) and you can place your heater in the sump so that you don't have an eyesore in your display tank.
