Up front I'll just say that this is my subjective opinion and by no means gospel. I'm not a professional photographer but I've been taking pictures for decades including darkroom work, so I do have a strong working knowledge of the settings and process that will matter.
Key things I'd look for:
- A good 'Manual' mode
- A macro function
- Ability to shoot in RAW image format
- Built-in flash
- Tripod mount
- AA or AAA battery operation
I'm usually not a brand loyalist and use whatever brand provides the features and function I need, no matter what the product is. That said, I've preferred Canon cameras for decades because they've got the right feature set for small studio (aka: miniature) photography. I've tried other brands and was always disappointed where Canon just gets it right every time.
Notice that I didn't mention resolution. Put simply, it doesn't matter. Unless you're interested in blowing your images up to wall poster size any modern camera resolution will be more than enough. I guarantee if you're going to take some time to resize images and do a bit of cropping (highly recommended) you'll be shrinking the image down from the beginning resolution. Heck, save money and buy an older model if it's got all the practical features that are more important than resolution. I'd also say the same for getting too hung up on sensor technology or any major jargon intended to sell product and not really much else; unless you're a professional who will demand a crazy high standard the technology today of any reputable company should be more than enough for your needs.
The vast majority of cameras today will be powered by proprietary battery packs; they're easy profit for a company (markup on them is nuts!) and a simple way to create planned obsolescence in a product (discontinue the battery and the camera needs to be replaced as well, isn't that convenient) so companies are all doing it now. If possible, a camera that operates on AA or AAA batteries is very useful because it's so easy to have a large supply of batteries on hand and swap them out. Otherwise, do yourself a favor and get an extra battery if you don't want to be stuck as a battery dies at a bad time.
Check the images of the camera, be sure it has a tripod mounting screw hole in the bottom. Most cameras will have them, but some point-and-shoot cameras are meant to be kept in a pocket and never mounted on a tripod. Even if you don't use one now they are very easy to get at a low-cost so it's a logical piece of equipment to get and something I'd consider all-but mandatory really.
Having a flash isn't critical if you're willing to light your subject well, but if you can't get the best lighting they can compensate sometimes. Not as useful if you're getting in really close but a nice option to have if you're shooting a larger group of models. Even with good lighting sometimes a flash helps the contrast and/or colour with the minimal effort of turning it on.
Most modern cameras will have the option to take RAW images which will be larger but more colour accurate. I'm currently using an older camera that can't shoot RAW images and it really does tend to oversaturate and alter the tone of some colours and that's a known side-effect of the compression that's used. Again, this is subjective and depends on how hung up you are on colour accuracy, but having the option, even if you don't use it, is nice.
A Macro mode is a must in this case and zoom simply does not matter. Optical zoom is what you want in any case, but it's moot with miniature photographs; it's all about the macro. Without a Macro mode, the camera can't focus when you get in nice and close so you'll need to shoot from a distance and crop lots of dead space. Getting in to showcase details also becomes all but impossible without Macro.
The real key is if the camera has a Manual mode, and for me, this is where Canon leaves other cameras behind; even if they do offer a Manual mode, it's usually not as robust and functional as Canon cameras. Auto mode is still useful and you'll shoot with it all the time, but when you really want to put a camera on a tripod and get some lights around a model and get in close to really take some good photos, you want a good Manual mode so you can control the settings. It's not as intuitive as using a 35mm camera but Canon gives the user access to all the key settings that will offer you complete control, even in many/most of their point-and-shoot 'PowerShot' models. Note: it may seem daunting, but really it's just a few settings and a general understanding of a few concepts; I'll try to gloss over it quickly but anything I'm vague on can easily be added on with a Google search and a bit of research about photography.
The key functions that Canon cameras let the user control that matter to me are, the Auto Focus, ISO, Shutter Speed, and Aperture Size (F-Stop). There are other ones that are also nice but they're not as key as these four.
Auto Focus: AF is usually a great function and it get used most of the time. However, it's also stupid sometimes and simply refuses to focus correctly, especially at short ranges on in complex compositions. Being able to turn it off and adjust the Focus manually in those times that you want to is very useful.
ISO: This controls how much light is needed to create the image; this is a setting that the camera will change considerably when in Auto mode in order to take good photos in different lighting situations. A low ISO setting needs more light to create the image (important in conjunction with the next two settings) but it will also produce a better image that is noticeably sharper; high ISO lets you shoot in low light but the images become noticeably grainy for all sorts of reasons that don't matter here beyond the fact that you want to be able to manually adjust the ISO so it stays low. You will adjust the light with the other two settings to work with the chosen ISO, as well as control the depth of field, but more on that in a second.
Shutter Speed: How long the digital shutter stays open to expose the image. This is one of the key settings to have control over the Depth of Field in an image (read: how much of the image will be in focus); a small Depth of Field and the background and foreground are out of focus and a large Depth of Field will have the whole scene in focus - sometimes you'll want one and sometimes you'll want the other. When you get in close to miniatures it becomes a bit trickier to control the Depth of Field because of the short distances you're working with and slowing the shutter speed so you can let more light in is key.
Apature Size (F-Stop): Also a digital setting in today's cameras this is what the aperture used to do in mechanical cameras. It's a tug of war between this and Shutter Speed when photographing miniatures; you want a small aperture (read: less light) to get a nice large Depth of Field so all of the model (and maybe its companions) is in focus, but that means you need to slow the shutter speed down so much to let light in that any handshake will ruin every photograph. Solution: mount the camera on a tripod, push the F-Stop all the way up, and then use the Shutter Speed to get the right light level. A very slow shutter speed will be needed so a tripod becomes a necessity.
With the odd terminology and the sometimes not-so-obvious effects that some settings can have it can deem a bit daunting and hard to grasp, but once you take some test photos with a basic understanding of the settings and then experiment a little it quickly becomes obvious and simple to do. Make sure whatever camera you get lets you control these basic functions in Manual mode and you'll have the control you're looking for to take photos of this kind of subject matter. It doesn't need to be a high-end DSLR with a large price tag if you can control these settings manually and mount the camera on a tripod.