I agree with the fact that Target's brand (Circo) runs small, so you will want to go up a size with them. I think the other posts have covered your size questions. I think (although not sure since my daughter is still wearing 18 months and she's 2) that 24 months is a wee bit smaller than 2T, but probably not much of a difference there. I don't actually think I've ever seen a 1T size before, but I would imagine it's the same as 12 months, so if you see one, that might be what you're looking for!
Good luck! Also, Children's Place is great if you can find stuff on sale and they almost always have coupons. Carters can be expensive unless you find their stuff on sale, but when they do have sales, I go there because I LOVE their stuff. Also, Kohl's carries Carter's brand stuff for lots cheaper, so you could check that out too.