The main consideration with these is the frequency of operation as some bands are illegal in your country and you need to know which ones you can use.
The available bands are 900MHz, 1.2GHz, 1.3GHz, 2.4GHz and 5.8GHz. The best penetration and range always comes from the lowest frequencies but the the radio spectrum is shared by a whole bunch of other people for all sorts of different things.
In general though you will either need a license or not be permitted to use the band at all until you get to the 5.8GHz band.
In this band there is not as much radio clutter yet but it is increasing. This is the band that the racers use so it is commonly called the racing band but any drone can use this band. It does seem to get an increasing traffic of WiFi equipment ranging from internet use to baby monitors so you’ll have better luck with these if you aren’t close to homes.
In the USA 1.2GHZ & 1.3GHz are also usable but again the radio clutter is increasing rapidly.
With any of these systems you have power limitations but these really only affect people who make their own systems as all the commercial ones are already set to the limits they are allowed to have in the country they are designed for.
One other consideration you should think about is antenna size. The higher frequency you use the smaller, physically, the antenna can be and still work efficiently. This means that you should take this into account if you are designing your own drone so that the size and lift capabilities of your drone can manage the size antenna you want to fit.
It’s probably best to join a club or an online forum to find out from those who have experience in these things to get the right advice for your local area.
It should be pretty obvious that the camera transmission frequency should match the receiver frequency and this means that you probably can’t mix and match system components.
However there are also considerations on the type of camera to use. While you can use any camera that is physically small enough and can stream video it is generally accepted that you should use a specialised camera for this purpose.
When you get to this decision you need to choose between CCD or CMOS technology. CMOS is cheaper, stye use less power but the resolution isn’t as good.
Then you have two main video systems in use. NTSC is the one used in the USA and some of Asia. The resolution is lower but the frame rate is faster so the image isn’t as good but it is smoother. PAL is the system used pretty much everywhere else. Make sense that the resolution is better but the video isn’t as smooth as NTSC.
Use whatever your TV uses and then you will be able to record the video and replay it on your TV.
As a starter you can use an integrated camera such as the GoPro but ultimately you will want to have a dedicated camera as their response time is much faster when flying from a light to dark area and vice versa, very important when you are flying fast.
This is unlikely to be an issue with one of the cheap fpv drones but will certainky be an issue with one of the more expensive but much better quadcopters.