If you create a save file at the Duke just before you fight Miranda, you can sell all your items except one gun, then fully upgrade this gun and buy any missing customizations from the duke. You will now be able to purchase infinite ammo for this weapon with CP in the bonus shop. Simply continue to reload the save file and do this for all available weapons (most but not all) in your inventory, and you can unlock the customizations (so long as you didn't miss any in the story) and weapon upgrades with 1 playthrough.

Protip: make a save file at the Duke with everything in your inventory already sold except your weapons so you do not have to sell your ammo, treasure, food, every time you reload the save file to upgrade a different weapon.