Nabeel,
You are best to run the chosen EX camera in 240vac. This way you will not have any issues with the very large copper cables you need to run over 500m for 24vac considering volt drop and current draw (very large conductors required for over 500m at 24vac assuming 3amps draw). I am in an IECEx country and pointed customers down this path and not the low volt path. There are many manufacturers that make Ex cameras that have the ability to accept 240vac onboard. You will find it incredibly difficult to find an IECEx certified transformer or power supply to suit your requirements.
Also as far as comms and video goes the best way to get an EX camera (PTZ or Fixed) with the fibre optic modem on board as well (yes you have to run fibre optics!) If you are in an IECEx country (and I suspect you are) you will find it very difficult to find a separate IECEx certified fibre optic product to plug your camera in. You cannot simply get a fibre modem and place it in an ExD or ExE enclosure. The basic rule with IECEx is the complete product has to be certified as a whole. You may remove items you do not need but cannot add anything that was not originally part of the product. This means it was never tested and hence uncertified. The exemption to this rule is to get the government electrical inspector to exempt this and they may allow this (or even ATEx products) but I have rarely seen this signed off as it means RISK. Sounds complicated but there is enough manufacturers products out there that can easily satisfy your project and requirements.
Oh and you will also find these cameras will be all certified for +60 to -60degC. check out Pelco, Hernis, Bosch, Oxalis, Videotec just to name a few.