I merged your threads. In the future, please put questions together in the same thread AND post, it makes browsing the forum and answering your questions much easier.
A header tends to make engines sound raspier, and the QR25 is definitely subject to this rule of thumb. A good catback exhaust with a large, high-quality resonator will cure it.
Try junkyards for the mirror. Call around, most junkyards are part of larger networks that can ship parts in from elsewhere. Otherwise get one from a parts retailer, though you'll have to get it painted.
http://everythingnissan.com or
http://courtesyparts.com
The best maintenance help is the FSM. You can get one from the link in my signature.
The transmission should last through the useful life of the car, at least 200,000mi if it's taken care of. A transmission swap is more trouble than it's worth, the cost of parts alone far exceeds the cost of simply getting an SE-R with a stick already in it.
Welcome to NICO!