I work in the site development world, on both large- and small-scale developments, etc. You're looking to create an emergency vehicle operations center, or an EVOC. I was on a design team that created an EVOC for a technical school. It included a 1-mile training loop & skid-pad, three bermed shooting ranges, fire training structures & classroom facility. I also reviewed plans for two more facilities that weren't built.
Most law-enforcement training facilities are between 0.7 and 1.0 mile in length, typically 24-28' width throughout, with various portions of course that are curved, straight, divided highway, on-ramp & off-ramp scenarios and a large central skid pad area. Most skid pads are rectangular and totally paved (i.e. not a donut) so they're more flexible for training. Some EVOCs are also set up for heavy truck training and include faux docks, blind corners to back around, etc. Typically EVOCs are constructed using asphalt (at least here in the Midwest they are.) Pavement profile is heavy-duty throughout, usually 4" of asphalt (one 2.5” binder lift and a 1.5” wear course) over 12" min. of compacted stone base. Thicknesses of asphalt and base are a function of vehicle loads, both static & dynamic.
It sounds like your client is looking to create a plus-sized EVOC that will be built longer and will need to tolerate higher weights & loads. You'll need to determine the various training scenarios desired and how they can be addressed by the course's design. IE, will the entire course be paved, or will you have areas of transition from paved to non-paved (dirt, stone, water obstacles, terrain / grade changes, etc.) Vehicle load calculations will be a must. Aside from the course itself, you'll also need to think about support structures on-site for training, vehicle maintenance, possible on-site fueling, parking for spectators, lighting for night operations, access control, etc.
You say that a civil engineer is involved; they should be able to provide a lot of guidance relative to design process, site disturbance, pavement profiles, etc. As well, some serious geotechnical investigation will be needed. Different soils have different bearing capacities, and a geotechnical engineer should be able to provide recommended pavement thicknesses as based on soil profile & anticipated traffic weights. It’s one thing to design a course for squad cars, but any military / armored equipment is going to have a whole different set of criteria.
Since the property is military-owned, do you need to go through any municipal plan review, local DNR involvement, engineering review and/or stormwater management approvals (depending on your locale)? All will demand attention and potentially lengthy approval processes.
With all that said, good luck. Crazy clients are sometimes the most fun to work with. The impossible becomes possible when someone with money & persuasion gets it in their head to make something happen.
/edited since I see some add'l information has been shared