It shouldn't be impossible to buy Attachments that already have Mods made to them. In many cases, this can represent different models of the Attachment. If this is allowed, how should it be handled?
What I'm looking at right now is that the credit cost for the Mod should be doubled (so 200 credits for the first Mod on a weapon/armor Attachment or 2000 credits for the first Mod on a starship/vehicle Attachment). The Attachments with Mods are more expensive but are also risk-free and don't require any particular expertise in Mechanics. For the DIY lovers with good Mechanics, they can be happy with the money saved if they do it themselves (especially if they have Gearhead).
Additionally, each installed Mod increases the Rarity of the Attachment by +1. This means that highly tweaked Attachments that come with a slew of Mods might be exceptionally hard to locate.
The other advantage here is that Attachments with Mods now have a sale price and Availability that represents the risk that went into customizing the item. This is useful when selling components, and means that a Mechanic with Gearhead can make a profit from applying Mods and then selling Attachments.