Plug-ins extend the functionality of Wordpress websites. For example, if you install an E-commerce plug-in on your website, your Wordpress site can become a full fledged E-commerce store with all the functionality.
Plug-in Development is difficult, and much harder than the theme development.
If a plug-in is not developed well, there is a danger that your Wordpress site might be hacked. There are thousands of free Wordpress plug-ins available, so you can take any of them and customize it to suit your needs.
Before you chose a company for your Wordpress plug-in development, make sure they have the capability to do so. We, at OmkarSoft, have developed several plug-ins of our own, and we know how to develop a secure plug-in which has the functionality required by the client.
Wordpress plug-in developer must be aware of the Wordpress Architecture. Below is the Wordpress architecture diagram
Tips Wordpress Plug-in Development
Wordpress has 30,000 free plug-ins, and I am sure that you will find the one that meets your requirements perfectly. If not, then I suggest that you search for a plug-in that meets most of your requirements, and customize it. This is a good place to search for plug-ins → wordpress.org/plugins/
Wordpress exposes many of the functionality using Hooks, Actions and Filters. Check how you can reuse them, a detailed documentation can be found here → codex.wordpress.org/Plugin_API