I've been working with OpenGTS and GTS Enterprise (GTSE) for three years almost exclusively, and have installed, configured, and made modifications to the code many times.
You should be informed that GTSE does support OBD2 without modification to the code. It also has additional features, and enhancements that OpenGTS does not. Both OpenGTS and GTSE are written by the same software developer, and share the same basic code.
In my honest opinion, the best path to success is to purchase the GTSE package, then install or upgrade your current OpenGTS installation, simply because it would eliminate any chance of codding errors, reduce development time, provide the ODB2 support (and reports) and additional features that OpenGTS does not have. Last time I checked with Martin at Geotelematics GTSE was $1350 USD
This bid is to install or upgrade OpenGTS to GTSE, and does not include the cost of the GTSE software, only the labor perform an install or upgrade. Quite frankly I could do it for €350, but the Freelancer site won't let me bid a price lower than €750 which is the minimum for the project.
You will also need a Device Communications Server (DCS) to communicate with the trackers. The DCS is server that runs separate from the OpenGTS/GTSE web app, the code varies depending on the make and model of the tracking devices. Some are supplied with GTSE others are optional, or could be developed, if needed.
I need to know which devices you plan to use to provide a firm price.