In general, the project you are outlining is quite simple. You can probably do it all yourself, with no collaboration needed.
One question though: I thought NMEA2000 was for industrial equipment designed for 24V equipment? I would double-check on that, since 24V would be enough to damage a Carloop, which is protected up to only 17V (ie. meant for 12V cars). If there is an issue, all you have to do is re-design the power regulator and the protection circuitry; not overly difficult to do.
You have a few options to achieve what you want. The simplest option is to buy a Carloop, de-solder the OBD2 connector and add your own connection. A slightly more complicated option is to use the hardware design available on GitHub and build your own, minus the connector. A third option is to modify the hardware design (again, available on GitHub) to include your options (such as a battery charge controller) and build it yourself.
For GPS, I would suggest NOT using an option from Carloop. Instead, use a proper marine GPS that has a serial interface. (The Carloop GPS also uses a serial interface.) You will get a much better GPS signal this way. The Carloop can get the GPS data over a serial connection, and you can use the information as you wish.
What do you want for battery monitoring? If you want only a simple voltage display, the Carloop hardware already has that. If you want battery charge status or to interface with a charge controller, then you would need to design this part yourself.