I don’t think the 70D is going to be worth the upgrade. I think you might want to figure out why you need to upgrade your T2i, and then look for a body to match that. Is it better low light performance, video, or sensor size? The 70D is a great camera, but I don’t think it’s that much better than the T2i.
The 24-70 is definitely worth it. I’d also argue that your 18-200 is probably limiting you (not great low light performance, weak dof/bokeh/creaminess). If the 24-70 will break your bank, look into the 17-55 f/2.8 (also a fine lens). Good luck.
If you use a cropped body, a 50mm 1.8 is probably your best bet. (Scales up to around an 80mm, and they’re about $100 a pop). You could also look into an 85mm or even a 60mm (I think Nikon makes theirs in a macro version). The 85mm is probably the best for portraits, but you can get great stuff out of all of those.
Hope this isn’t too late.
I don’t think the 55-200 is well suited for low light and I don’t necessarily think renting a 51/5200 is going to change much. I’d probably rent a 5D or D800 and maybe a 24-70. You should probably have a 70-200, but you can get away without it. An 18-105 will also have poor low light performance, but does cover a decent range. I don’t think Nikon makes a 55mm prime, but maybe look into the 50 1.8. Hope this helps.
Scott Kelby’s The Digital Photography Book is great. Really down to earth advice and tips. Popular Photography Magazine also has some good stuff for learning tips and tricks.
However, I wouldn’t get too bogged down in reading all the technical stuff, though. Obviously you need to learn the basics, but even more importantly, I would look at other great photographers and artists. Don’t worry about how famous they are. Just go to a bookstore or library and browse through. Look at work that you like and work that you don’t like. Also, check out some books in the art history section. While the tools are different, you can still learn from the visual storytelling techniques.
This is a tricky one since the 55-200 is really not made for low light. You have the right idea, so play with some practice shots and see how high you can kick the iso.
Honestly, I would try to see if you can rent or borrow a 50mm 1.8 (or if you can, 1.4). It would probably be pretty inexpensive. You might even be able to rent a different body, but that might not be necessary. Check out Borrow Lenses or any local place.