That's a cool place for a tank. I have a 24 inch deep 20 gallon hexagonal aquarium with a current usa 18x2. (36 total watts) I am able to Wisteria, java fern, some kind of hygro, pennywort, and a few others fairly well. This is definitely low light, but if it is a real pain to get to, then maybe that is not a bad thing. I have had some plants that just never did too well and I doubt any of them would win any awards, but I enjoy them and hardly know any better.
I am also using a red sea wood air defuser (I guess it isn't a wood air stone). It seems to work fine, but you do have to clean them every now and then and it is difficult to build up the pressure to run them of diy co2 (however I am doing just that). The wood difuser is cheap they are like 3 for 5 dollars, and you can find them at the big pet store chains.
I have trouble keeping nitrates in the tank, but I have a pretty thick sand bed and not many fish. I feed heavy and supplement with some Seachem Nitrate.
I don't know about a nice carpet plant, I've got some e tennelus and some microsword in mine, they definitely not stellar. - they are growing though. I am also not too sure of the spiral cfl's. I've not tried them. I am also not to sure of an undergravel filter I've heard they cause root related problems, but i've never tried that either so I can't honestly say. I would think that a canister filter would take care of you though.
All that to say: it can be done. I am in school and can really only afford to THINK about all the pretty things I want to buy for the fish tanks. I have really enjoyed my low light-low tech-low cost tank though. You may have to accept that some stuff won't grow well in it and you may end up frustrated half the time, finding yourself constantly scheming to make the tank better, and dreaming of more and better tanks; but if you didn't do that, you might forget that you have a hobby of trying to grow plants underwater in your living room.