On the D5100 the Nikon 50 mm f/1.8D will not autofocus. You will need to purchase the "G" version for autofocus on the D5100. Only lenses with the designation AFS will autofocus on the D5100 which does not have a motor in its body. With respect to 50mm vs 35 mm you may find that the 50 has too much reach for normal use on the D5100 which is not a full frame DSLR. On the other hand it will work reasonably well as a portrait lens since it will have an equivalent reach of 75 mm on the D5100. Both the 35mm and 50 mm "G" lenses will produce images of roughly equal sharpness.
Yes. And with a bigger aperture, in other words a F1.4 or F1.2 --50mm (18-55mm) if you can find it but it will cost you. For low lighting conditions you want to have a compensate with the iso (speed light sensitivity).
SD cards are a dime a dozen. I used adata 16gb class 10 which has bad review on amazon but works fine for me. I have 4gb scandisk extreme, 16gb kingston class 6 all used on my D7000 (not d5100) and they all perform the same personally. Except the class 10 download a tad faster on the computer.
That sounds like a pretty good deal if you want to change out your lenses between taking pictures close up and far away. I's probably just buy the camera body and get you an all in one lense 18-200 or 18-300 lense Nikon or compatible Sigma lense like i did and your set to take pictures whenever the moment arises and not have to carry around an extra lense. Just do a little shopping around and you can probably come in about the same or even little lower for an all in one lense to go with the D5100 or D5200 body.