Welcome, KommMaster!I know it's not the quick answer you're looking for, but have you searched the archives? There's gobs of info and advice and pics and video links for silencers, filters and seating for the C5.
I like the air box that St. Beardface posted you can source the parts from hardware and auto parts stores. Have not tested it myself but I like it!! http://www.enfieldmotorcycles.com/forum/index.php/topic,12468.0.html
For your seat, I just got the Corbin style 2 up saddle and I'm a fan. It's firm (very firm) but also a good ride. Definitely big enough for you and your woman.
If you want loud, get the upswept cocktail shaker although all of the "silencers" are significantly louder than stock. http://nfieldgear.com/enfield-store/upswept-cocktail-shaker-silencer.html
Does the bike require mapping of the exhaust fumes?