Attachments use what's best for you,watch driftor in depth videos like the one on grip and suppressor as they do different things and affect different guns err differently lol but you may be wasting an attachment slot.
When you prestige it's basically resetting back to start with unlocks but weapons stay at their respective level that you got them to previously and it seems the attachments you unlocked stay too (they did for an and kuda) you can also choose a permanent unlock go for something you unlock at a high level.
Also choose perks that go with your plays type or are situational you can change to I have one that has blind eye cold blooded and engineer (or I did before prestige) if opposite team throw a lot of scorestreaks down they won't get me and I can destroy them.
At the moment I like fast hands ghost and hard wired,I'll swap ghost for flak jacket for objective modes.