The trick is in learning how to link triggers AND events. You can set up the triggers for lower fuel and ammo, then set up another trigger for the player entering the area starting position and speed at 0 mph. Then you link it all into one event, like "Take-off, low fuel/ammo" with a 1 second delay.
The best example i've ever seen of this kinda thing is Guy's Milne Bay campaign - Ferry 01 mission where you get an engine fire on take off immediately after reaching 70 mph on rollout and you have to ditch (or crash land) successfully to survive for the next mission, which is a Ferry 02 second attempt. Even though he uses different parameters in his triggers, the concept is the same as what Merlin is trying to do here - set up the mission from start with configuration deficiencies in the player's aircraft.
If you have Guy's Milne Bay campaign with all the many parts properly installed, you can open and analyze this MB01 - Ferry flight in Mission Builder and see how he set up his triggers and events. A close inspection of the events and triggers shows that he could have set the fire to start as soon as the player spawned into the mission start position.
This can be done, as Guy Boullenger has already proven...