Save ur money(GP) until you become sergeant or staff sergeant...
Once u become the following ranks u will get bonus GP:
Private: 10,000 GP
Sergeant: 20,000 GP
Staff-Sergeant: 30,000 GP
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When you become staff sergeant you should have about...115,000 GP total
now the weapons you should buy are listed below, they are the best guns in da game:

Best Sniper: AWM Price: 56,000 GP

Best Machine-Gun: M60 Price: 49,000 GP

Best Rifle: AK-47 or SCAR Price:45,000 GP (each)

Best SMG: MP7 or MP5 Price: 32,000-40,000 GP

Best Shotgun: Spas 12 Price: 42,000 GP

Best Pistol: Desert Eagle or Anaconda Price: 25,000-38,000 GP

Best Scopable Rifle: G36K Price: 48,000 GP
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Well I'm would buy an AWM and G36K....
Hope iHelped...