Naturally, use eBay,
Buy a Chimera
Buy a Cadian Infantry Boxed Set
Buy a Heavy Weapons Boxed Set - 5 different weapons - 3 of each. Build the Heavy Bolter normally, with the Commander from the Chimera - use some legs from the Infantry - hey presto! An Officer.
Missile Launchers - make from Missile Launcher arms and Cadian Infantry bodies
Of the whole infantry you have bought - including the Commander parts in the Chimera and HW Box = 27 men.
In total you have used - if you follow my instructions exactly - you will have around 20 men left - 10 for a squad, and if you want, another infantry squad for your platoon.