My only real complaint about the lower-end DSLRs is that they are physically too small for my hands, based on playing with them in camera stores. The middle-range and high-end models give you more external buttons for quicker setting changes without going to the menus.
---------+---------+---------+---------+---------+ Joseph K Seattle, WA, USA
I concur with the menu concern. I'm working with a Canon sx160is point and shoot with AV and Manual modes. I have it mounted on a homemade tripod head with a homemade cable operated shutter trigger. Due to where I had to put the post, I have to operate the menu and the buttons with a stylus.
Getting a camera with R/C (radio control) shutter release would be suweet.
Welcome to Nikonians! The D3200 is significantly more capable than even the Pro film bodies back in the 1990's. It has excellent resolution and dynamic range, greatly improved over the preveous generation of DSLR bodies. While it does have limitations such as, no sub-command dial, and dedicated buttons to quickly change settings without having to go through the camera menu. This might slow you down a little but unless you shoot sports or action, it shouldn't be too much of an issue.
The oversimplified difference between the 3200-5200-7100, all of which have 24 Mpixel DX sensors, is that you get a LITTLE more of everything as you move up the line: a little better AF module (11 vs 39 vs 51 AF points), a little better viewfinder (pentaprism vs. pentamirror, better viewfinder magnification and coverage, etc.), a little more in the way of dials and hardware vs. menu adjustments, better weather sealing, etc. etc.
Many of the features on the more expensive versions may or may not be that important compared to the extra cost.
A D3200 has all the basics you need to control composition and exposure (P, S, A, M modes, exposure compensation, a variety of metering options). It even has a lot of the in-camera nifty tools like active D-lighting, AUTO ISO with adjustable upper limits, Picture Controls, etc.
It's easy enough to compare features and specifications. Unless you absolutely need some feature that only the D5200 or D7100 offers, and don't object to the size of the D3200, the lower cost model may be enough to gain access to a 24 Mpixel DX body. If money is no object, get the D7100 and get everything there is to get in this series. If you can deal with less, the 3200 and 5200 offer alternative price points.
The higher megapixel is the last thing I look for in camera.A camera has to have the features that go beyond the megapixel number. Secondly it has to have the physical features that will make it comfortable to shot. Thirdly the ruggedness to withstand being used.
Then it seems your task is straightforward. Since all three cameras (3200-5200-7100) have the same sensor resolution, it comes down to what features you cannot live without, or whether or not a particular body is suitably comfortable or rugged.
If sensor resolution is not a deal-breaking issue, you could make an argument for a less expensive set of choices: 3100-5100-7000, which are IIRC in the 16 Mpixel family.
I just steered my nephew toward a used 3100, which is about as dirt-cheap a way as possible to get a really nice DSLR to learn with. Beats the socks off my ten-year old D100.
I don't know how the 3100-3200 series cameras stand up in practice, but my D100 and N80 cameras withstood a ton of outdoors abuse: heat, sand, dust, humidity, rain, clanking around on my hip while scrambling over rocks, ice, brush, etc. I'm sure someone on this site can testify to how much they have abused a 3100 or 3200.