OK, I have an idea for a cool mission / campaign but need help

[Patagonic]

I idea is the following: The entire Sahrani island is in control of the SLA forces, the political situation is to delicate for a full invasion for the moment. Nevertheless a secret operation to attack SLA forces has been autorized.
Only one operative will be inserted in the island, the mission of this operative will be to sabotage and cause as much damage as possible to the enemy infrastructure.
So the idea is to have the entire map populated with SLA forces (vehicles and tanks patrolling the routes, enemy bases full of soldiers, checkpoint in every mayor towns and cities and choppers looking for you to kill you).
Your only weapon at the beginning of the game will be an M24 sniper rifle. You will have some idea of some places where the enemy will be but you will be able to encounter by surprise enemies soldiers or tanks patrolling or camping anywhere.
The objective of this mission will be to inflict as much damage to the enemy forces as possible and stay alive. Use the SLA weapons that you could find against them. Study the situation and attack / ambush at the right places and times (night or day). Stay cover all the time, they will be looking for you!!
You will have to be very patient; this mission could take you and entire month to be finished (of course it won't be necessary to kill every enemy, mission will be accomplish when you decide to leave the island, of couse if you find how to leave the island!).
O.K. now a few questions:
I have a few vehicles patrolling routes in a "cycle". What would happened when they run out of fuel? The will go automatically to the next fuel station, refuel and then continue to their next waypoint?
How does the engine of the game work? Every single extra unit affects the game performance - FPS? Or it depends of the number of units in a specific area next to the player?
I need to know how to make a trigger for something like this: when a unit is kill another unit that was inside a camp move to a special location (waypoint). Can anyone help me with that?   

[D.murphy man]

Populating the entire island would indeed cause some heavy performance hits. For such a large dynamic mission id recommend you look into using DAC: http://www.flashpoint1985.com/cgi-bin/ikonboard311/ikonboard.cgi?s=9a39c4d7f9dba77a0662a994e96d0efb;act=ST;f=70;t=70582;st=0
With this you can set up zones, and randomized camps. When a player enters a zone it will spawn any thing you want, soldiers, vehicles, helicopters, and they'll all randomly patrol. Theres literally a ton of features and possibilities with DAC that i cant go into here, but check it out, its excellent set of scripts, MP compatible too.

Also the mission idea you described would sound more suited if the player was a resistance fighter/rebel/terrorist what ever you want to call it, that was instead supported (secretly due to sensitive political situation) by the US government or something. Players could arrange secret supply drops by boat from US SF of the coast, relay intelligence and information back to US spies or by stolen radio equipment, etc..

[Wolfrug]

This is basically OFP's Abandoned Armies that you're describing, except there're two sides and you get lead a resistance group. The spiritual successor of AA is Punishment Battallion by Lee Hunt, which you might very much want to check out.

As to making it yourself, D.murphy man there has given you a pretty good starting point with DAC, although it can be a bit challenging for a scripting novice to get their heads around it (nice tutorials come with it though!).

However, you probably need something more than just SF action behind enemy lines ambushing convoys and blowing up buildings: but as it were, this is more of a Mission Discussion kind of topic when it comes to actual content. You could start a separate topic there if you want to discuss those aspects more in detail.

As to questions:

Fuel: Not AFAIK, no. However you can refill the fuel tanks yourself with the setFuel command (look it up in the comref).

Units: More units will cause more latency, but ArmA is also streaming, meaning units outside of your visual range won't have as high an impact on performance as those right next to you. Nonetheless, it's always a good idea to use some kind of spawning/despawning script (which DAC comes with by default) for really big missions.

Your last question I don't quite understand: you mean if the player (for instance) kills a unit which belongs to a certain group/is in a certain area, then the player is given a new waypoint? It's a bit complicated, so I suggest you read the section on triggers and waypoints in the Beginner's Tutorial to get a better understanding of how things could work out.

Good luck!

Wolfrug out.