Infiniti recommends using premium grade fuel in the M45 and also in nearly every other car they make.
location of crankshaft sensors in 2004 infinity M45
You buy a new one!
One can purchase a used Infiniti M45 on Car Gurus and Autotrader. If you don't trust purchasing cars online, then you can go to your local car dealership.
The cost of a used Infiniti M45 varies depending on how old the car is, body condition, mileage, and engine condition. Consult Kelly Blue Book for specific prices.
The factory 2003 Infiniti M45 shocks are monotube. It is evident by the huge piston size. For a premium luxury sports sedan as heavy as the M45, monotube shocks makes sense as they provide stable control on longer drives on different road conditions.
Unless you can feel it running rough and knocking - no.
The oil capacity for a 2007 Infiniti M 45 is approximately 5 quarts of oil. The oil can be purchased at any auto parts store.
The cam position sensor for a 2003 Infiniti M45 will cost about $30-40 for an aftermarket brand. For an OE (factory) part, your best bet is to call the Infiniti dealer for an exact quote. Theoretically it should be in that same price range, although it's not uncommon for OEM parts to be more expensive than aftermarket brands.
The 2013 Infiniti JX runs on premium unleaded (recommended).
The 2012 Infiniti QX runs on premium unleaded (recommended).
The 2013 Infiniti M runs on premium unleaded (required).
The 2011 Infiniti EX runs on premium unleaded (recommended).