Hey M, welcome to the forum! You started with a great one - way above average in my experience with other forums. Believe it or not, this forum right here is part of the FX ownership experience. The ideas, support and community connect you to your car in a way I've never really experienced before. It's pretty cool.
Here's some good info for ya - the 2003-2004 models were the initial 1st gen release. In 2005, there was some minor re-styling of the exterior trim and interior. Make sure you know which one it is you are getting if you have a preference. Of course, no matter which interior you get there are aftermarket mods/changes that can be made.
To my knowledge there are no available Supercharger kits for the FX45, so if you want to buy a kit and have a plug and play supercharger, stick with the FX35. The FX35 shares the same engine and drivetrain with the Nissan 350Z and the Infiniti G35 so any performance modification those cars can handle usually translate to the FX with some minor customization, if not a direct bolt on. Take advantage of the years of history with the VQ35DE engine in the FX35.
Be realistic about the cost of major mods to the car. While they provide a lot of enjoyment, they require time and attention as well. They will always cost more than you planned and can continue to add maintenance costs down the line. Read up on fuel costs as well - in these times more than one person has said goodbye to the FX in favor of a more fuel efficient commuter.
Good luck on your purchase and we all know you'll love the car. Welcome to
