How to start gaming a scenario?

I am working on a scenario and I am looking for advise. I have imagined a setting in the future where inadequate water supplies has lead to national governments and international corporations buying land in other countries to claim water rights. The water is then transported back home -- as water or as foods, eg wheat, rice, etc -- or sell it to the highest bidder. This lead to armed insurrection by the locals even though the water claims are legal. The insurrections have now devolved into multi-party factional/clan warfare. All factions have advanced military equipment (mechs!), but some factions have better supply lines and maintenance facilities than others.

The environment is harsh. Dust storms of national and global scale have made long distance wireless communications unreliable. So, despite the advanced equipment the factions continue to rely on scouting (human and autonomous drones) and runners to convey situation awareness and instructions between units. Even long range targeting and retargeting takes significant time, but once established is accurate. Some factions rely on well planned actions with strict adherence to the plan by the discrete units. Other factions rely on discrete units making local decisions stemming from the unit's overall purpose.

The scenario I am working on is a tanker truck needs to be driven to the coast (ie, off board) by its corporate owners. A rival faction wants to divert the trunk to its own facilities for local distribution (ie, driven off the other side of the board). It is a stalemate to have the trunk not reach either of these destinations before N rounds of play. It is a defeat for both sides if the tanker trunk is destroyed.

My initial plan was to use the Horizon Wars rules as I played an early edition of them from a few years ago. The rules are basic, but do make for a quick game. However, I am not wedded to them so am open to suggestions for other suitable rules.

My first question is how to start gaming a scenario? Is there an approach to determining the units needed? Or is it ad hoc, ie just play a game with a guess at unit composition and respond to the results? Or something else all together?

Update: The working document for this is at "A setting for near-future wargames."