Make sure you look at the main site if you haven't already. Most of the information you need is there and is VERY helpful. Checkout
this link for startup costs. You can get away with only spending about $140 for minimal stuff.
Don't feel bad about buying him! He is with you now and you sound like a determined owner to get him a good home that he deserves.

As said above, at least you're doing your research, which is more than some people do when they buy a turtle all of a sudden. I am also in your position. I recently saved a turtle without realizing all the costs that are involved but I got through it with help from this wonderful forum and it's owners, so don't fear! I don't have all the answers to your questions but I'll try and help, based off the answers I have got.
He is small right now but will grow, as said above, to maybe 5" in a couple of years. Because of this, you will want to get a bigger tank NOW so that you don't have to spend more money later to upgrade to a bigger tank. I would also suggest a 40-50 gallon tank, that way when he's 5" it will still be enough room. Going to a pet store and buying new will be very expensive and most likely overpriced. I would suggest looking online, with the help of your parents, for a used tank on craigslist or any other website that allows people to post their stuff for sale. If you don't want to buy a bigger tank you can get a 20 gallon tank, however you will have to upgrade to a bigger tank quickly and in the end, it will cost you more money.
An even CHEAPER alternative to an aquarium is something called a stock tank. They are meant to hold water for animals (like you see on farms) so they are big, inexpensive and will get the job done. You can also buy big rubbermaid tubs. You have to make sure you enforce the sides and check it regularly as they aren't meant to hold water and can break easily. This would be your cheapest option. It doesn't look the greatest but hey, it gets the job done!
As said above, invest in the filter. This is what will keep the water clean so it's a really important one. The cost will vary depending on the brand and how big you get your filter. It is suggested that you get a filter that is rated three times the size of the tank. So if you have a 50 gallon tank, get a filter that is rated for over 100 gallons. The best bet for filters is finding a canister filter. This will be expensive and will cost you over $100 if you buy it new.
ZooMed makes turtle docks and this will be easiest to get at a pet store. I got a large one for $30 so they are affordable. It is also possible to get a little crafty and make your own dock, either on the tank or in the aquarium. Check out the Habitat - Indoor subforum for some examples of other owner's basking docks!
Turtles also like to swim so make sure, with whatever tank you end up getting, you fill it up as high as you can make it so that you can get the most swimming room.
I'm not sure what to feed baby turtles so I'm sure someone else can help you with this.

When I got my little turtle I made a thread and asked lots of questions just like you. I also got very informative answers in response so you might want to check out my thread
here! It might answer some of your questions.
Best of luck!
