I have wondered the same things. I use an In-Line hose, the standard barrel at the moment and the standard stock. I looked in to a sight/scope but if you get one it'll mainly be for aesthetic purposes (nothing wrong with that - I may still do it because it'll look ****-hot and put that bit more fear in your opponents), as the hopper will stop you getting any decent scope that you will see through your mask. Though if you're going mag-fed then you could go for the scope. I've played one game purely mag fed, and although you feel more of a sniper and use less paint, it sucks that you have to hide to change mags. I'd suggest getting Mag-Couplers so you can get twice as much shots before needing to change.
I've got a Dye Rotor hopper (because if I switch to 3-round-burst or full auto, gravity fed hoppers won't feed them quick enough), and just got a box of First Strike rounds, to go paintballs in the Rotor, and First Strikes in the magazines. Lastly I have custom Mag Tags (the plate above the magazine that says Dye).
I'm off to eBay to buy a Bi-Pod now I've got myself thinking!