You can use Steel Door Guild Coins to purchase weapons, armor, and accessories of each grade. These items may be restricted in terms of augmentations or attribute options, but they will be more than enough to help you on your journey.
Once you outgrow the equipment, you can trade them back for Steel Door Guild Coins again! You can also invest in the Aden Reconstruction project if it suits your fancy, or even gamble. Don't forget that Steel Weapon Packs and Steel Armor Packs have a chance of yielding Requiem, Apocalypse, or Specter equipment!