In order to purchase all 18 weapons and 10 perks (no need to buy the Equipment), you must first acquire all of the weapon blueprints throughout the campaign. You do not have to upgrade any weapons for this achievement, you just have to purchase the weapon for use. The perks are always available (no blueprints), with the most expensive one being Double Health for 20,000 Energon. The others are less expensive as you go down the list.
Look here for the blueprint locations:
NOTE: There is a perk near the top of the list that will give you a 25% discount on all weapons and weapon upgrades. It's called Cheap Shot. Buy this early to make things go much more quickly.
You get 20,000 bonus Energon for beating the campaign. After that, I would recommend just playing through the campaign normally and buying the weapons and perks that fit your playstyle, then just play some action-heavy levels on Easy to grab any Energon that you need and make sure to pick up the remaining blueprints.
The achievement will pop shortly after you purchase the last weapon/perk.