Will the merging with everything else from the PC account include credits? Ship modules? Existing weapons/armor Cmndr currently has?
No Merging.
The copy/transfer process will normally grant you a free horizons account with a new commander.
However, if you want to copy/transfer to an existing account, that account will suffer a Clear Save - so everything on that account in terms of game progress will be lost (ship, modules, credits, engineering, materials, etc)
Cosmetics are not tied to the Commander save, but the the account behind the save, which means you will keep your existing cosmetics