I would recommend getting a 5g w/ a heater, if possible, whether you're getting a coldwater or freshwater fish. That way, you won't have to cut out a place in the hood of the tank to fit the heater. ( I've had several 5g's w/o heaters, and the hoods on them would look difficult to cut into w/o destroying them) If you can't find a tank including a heater, small heaters rated for 2-5g tanks aren't that costly. Then again, a couple of the 5g's I have, have extra 'holes' in the hood in them, that way there's a place for the cord from the heater to run through, allowing the hood to close all the way.
(even IF you are planning on getting a goldfish, the heater would be necessary for treating for ich)
definitely wouldn't recommend putting the tank by the window, if you can avoid it. bottom feeders wouldn't solve the explosion of algae and the really green water you get when the tank is by the window. from the pictures of the 1g you had, it looks like you have room for the 5g in the same place with some minor moving around. but if you want the tank by the window, i would have the shade drawn during the day to reduce the chance of the tank getting a lot of algae.
I've never used carbon in any of my tanks, so I'm not sure how well or consistent it works. from what i've read, researched, and been told by other people is that it doesn't work that great, needs to be changed often, and if you treat a fish for disease, carbon filtration removes the medication from the water.
So....really sorry for rambling answers to your questions, hope it wasn't an overload. Again, I've had several 5g's and you're asking the same questions I once had

so good luck with whatever decisions you make, there's a lot of people in this forum that are pretty knowledgeable and able to answer your questions.