I know you know the tank is way too small. If you're having trouble affording something bigger, go buy one of those pond inserts you get at a garden center. I know I sound like a broken record, but I truely believe this is the best option to give your turt a lot of room for just a little bit of money if you happen to be pressed for cash.
Now about the lighting. The windows won't be enough because the sun is filtered by the glass. On top of that, the turtle needs to have the option to bask in direct UV light all day. The basking area needs to be about 10 degrees warmer than the water, and without a lamp directly over the basking spot, he won't get as warm and dry as he needs.
But it's not all that expensive to do it right, honestly. I wish you the best of luck

Everyone here is willing to give great advice if you need
