Yes, I've seen the two piles more to the middle of the level. Infact, I was building my second oil pump there.

After some time, the scavangers also went to those piles of scrap automatically. (After I've discovered that scrap, it took some time, not sure if the piles of scrap near the start position were depleted.)

I don't think the total amount of scrap is the problem. Perhaps I'm just not good at this game yet, however I do think you can get easily overrun.

You need scavangers to get scrap, when all scavangers are killed and your gathered scrap is too low, then you're uhm out of luck. Being able to sell some buildings could solve this problem, perhaps even spawn some troopers or scavangers after having sold the building you've selected to sell...

I will play it some more, and give some more detailed hints on my problem, I'm not saying it's too difficult, it just lacks something to get it more balanced.

Edit:

I can win the game, without making any vehicles ... I made 13-15 scavangers, had build up two armies of about 10 troopers + 1 medic or 2 flamers.

Still while building the required buildings, like for example the 6 water wells, the scrap amount never exceeds about 200. This means you'll never be able to buy trikes or buggys, also the oil takes a lot of time to even reach the 500 required for 1 vehicle. I choose to attack the waves before they went to my base. This works, however staying and defending your base instead, because you're building up your base, was what I was doing before.

When using the latter technique it's not very easy to win, infact, most of the time, your army will be destroyed and scavangers will be mostly be destroyed too. Since the scrap doesn't reach anything higher than 200 this will mean that in a last attempt to defend the base choosing either scavangers or troopers won't matter. They will get killed just the same. Troopers can't withstand the wave of enemies and the scavangers won't survive as long as even 1 enemy is alive, even if they can attack.

Like I said, this is when your scrap is about 200 and your army is destroyed ...

Cheers