The focal length of a so-called normal lens is determined by by the diagonal length of the sensor or film. You can calculate this using the formula f = SQRT( length^2 + width^2). SQRT = square root. n^2 = n squared.
For 35mm Film: f= SQRT[ 36 ^2 + 24^2) = 43mm usually rounded to 50mm for lens manufacture
For 6 x 4.5 cm film f = SQRT[60^2 + 45^2] = 75mm
For Nikon DX f = SQRT[23.6^2 + 15.7^2] = 28.3 mm usually rounded to 35mm for lens manufacture. Do the math an you will understand.
to calculate the crop factor between Nikon DX and 35mm film
43mm/28.3 = 1.52
For a DX camera with a 35mm lens the corresponding focal length for 35mm film would be 35 x 1.5 = 52.5mm A 35mm DX lens is approximately equal to a 50mm film lens. For a 6 x 4.5 cm the corresponding lens would be 75mm
I have always found B&H, Adorama and Amazon provide good service. B&H and Adorama have the fastest shipping. Many years ago I bought a light meter that was DOA from Adorama. They sent a new one immediately.
The camera that you have chosen is quite good. I would use it. DX sensor cameras with the 24MP sensor are fabulous. The difference between the various Nikon
series for DX cameras is the menu systems. The D7200 has more stuff available on buttons and a focusing motor for older lenses. Your camera is capable of a lot.
Read the Nikon USA website. I believe that you can even read the manuals on line. I have a android app that has all the Nikon manuals available. The DX sensor is better than film. I fondly remember my old Leica M3 however.
This focal length range is a lot of fun. There are a good few you tube videos that discuss this choice. Most like having 10mm lower limit and claim this appears much wider than 11 mm. Check the angular coverage. The edge softness and chromatic aberration varies between. It seems that the Nikon lens is the best here. I am considering trading my Sigma 10-20mm f4-5.6 for the Nikon 10-24mm. The upper end of the zoom range is interesting as well a the lower. The 10- 24mm range would be excellent for landscape and architectural photography. Both ends would be very useful for these purposes.
I am not sure why one would want to replace the Nikon 35mm/ f1.8 it is one of Nikon's best lenses and is quite cheap. I have some Yongnuo flashes and timer for long exposures and they are good products. I'll keep my Nikon lens.
The d 7200 is a great camera. The 18-55 mm lens a is a great general photography lens. It focuses quite close and is optically good. It is cheap as well. The 55-300 is a good lens but focuses a bit slowly . The 70-300 is a bigger lens but focuses more rapidly and is more expensive (there are a few versions of this lens. you want the expensive one. ) If you don't need the reach the 55-200 is a nice small lens - easy to carry and optically good.
There should be no problem if you get this from Adorama, B&H photo or the Nikon Store. The d7200 is a great camera.