It does a fantastic job! I went from a D3100, and like you found noise at low levels (800+). I own the D7100 now and greatly enjoy its performance at ISO 4000 and below. Also, the grain that you see at higher ISO is also dependent on you nailing the proper exposure. The better you nail it, the less noise you see in higher ISOs. Also the more light there is the less grain you will see at higher ISOs. Here is an example of an image I shot in a dark room @ ISO 4000 with the D7100 https://plus.google.com/104259763574908799227/posts/AbRQMh5uX1N
It is a great camera for beginner. You have to put some effort in learning it, read manual and get a good book, Dave Busch , dummies book, etc. you won't out grow 7100. 5200 is great camera, but only if your budget is limited id say get 5200. 7100 is perfect consumer/ next to pro level short of getting FX BODY like D600/610.
most all nikon lenses work on most nikon cameras since 1971. nikon as opposed to other brands came to the decision that their clients have spent a lot of money on nikon glass so they decided not to change their mounts as other cameras did. the D7100 has the same F mount as your 40X and uses the same 1.5x lenses. the D7100 is a beast of a camera and you will be so happy you got it.
It all depends whether or not you want to have a DX (APS-C size) or FX (classic 35 mm size) format sensor. The DX sensor has a 1.5X crop factor that will give your lenses a field of view that is 1.5X smaller than the same lens on a FX format camera. This can be an advantage if you shoot wildlife or sports, as it gets you "closer" with any given tele lens. However, getting a wide-angle lens is more challenging in the DX format, although there are still good choices. Both formats make great images. The DX bodies are less expensive, the FX sensors are inherently less noisy and theoretically capable of higher quality. You can use any Nikon DX or FX lens on a DX body. You cannot use DX lenses on an FX body without image cropping. I have a D7100, mainly because I started out with a D100 back when all DSLRs were DX format, and I built my lens collection around the DX sensor size. You won't be disappointed in either body--they are both pro-level cameras with a full suite of controls.
Do you mean "Grey Market?" Grey Market means it was not meant to be sold in the USA and most of the time not the original box with all the items that would normally come with the camera, like the battery, strap, etc. Most importantly; does not come with a warranty. "Black Market" is something usually illegal, underground, stolen goods. Amazon does not subscribe with Black Market retailers. This item is sold and shipped from Amazon and is a Nikon Product with USA warranty. If the lower price is your concern, Nikon just recently lowered the price of this camera. This camera is perfectly legit.