I have been working with OpeenGTS and GTS Enterprise (GTSE) for the last four, and one half years. During this time I have customized it in many ways, and written custom Device Communications Servers (DCS)
When it comes to the OS I prefer CentOS, that is what I propose we use.
As for customizing the front end, the UI/pages are written in Java, and .jsp there is no PHP involved what so ever, so any web UI developer you choose will need to know Java and/or .jsp
There are several mobile apps that will work with OpenGTS & GTSE available already, and one is available for free from the OpenGTS site. Also the latest version is mobile more friendly then the previous versions.
Other considerations are:
- What devices does your client plan to use?
- Do they need support for email or SMS alerts?
Something you and your customer need to understand is that much of the OpenGTS/GTSE configuration is defined in .xml and text conf files. The documentation is online for the most part, and of course there are extensive comments in the source code. Still if they are going to maintain the system they will need to have knowledge of ANT, Tomcat, MySQL and the Linux command line.
If you do want to break this up into two projects, I'll be happy to do just the installation, but you will need to create a separate project for that, as the $750 minimum bid is way more than I would bid for a simple installation.