you could try mike at MJP. They will be OEM parts but cheaper than the dealers. Also Holdcroft do 20% discount on parts. See forum discount for more info
Not a fan of single pipes. You have the issue of remodelling the rear bumper or a kit. Don't know of anyone who has one. Maybe worth asking on the American forums what there opion is or do some searching on 350z-tech
tickover once warm is just under 700 with the aircon on. Nearer 600 with the aircon off. All depend on how cold it is when you start your car and if its garaged.
10w-40 Silkolene Pro from opie oils is what I use. Around £40 for 5ltr with discount