Re: Which route should they go with Quicksilver's costume?
I'm not sure that they'll outfit him with a costume for this outing. If he starts the film from outside of S.H.I.E.L.D., then I don't think he'll have "earned" a costume yet - if he does, it'll be basic and should also account for the air friction of super speed...
If he starts out as a "project" from S.H.I.E.L.D. and is already a part of the team at the beginning of the film, then they'll go the Hawkeye route, and Pietro will make his own modifications to the suit to better match his personality.