First of all what are your long term plans ? If you plan on riding it for a long time stock then maybe some Gen 2 and shed some weight. Light weight battery, get rid of some of the idler wheels, dump the stock brake light and garbage rack, a front end kit if you wish (be carefeul what you spend your money on) maybe a Timbersled or not.
If you plan on Turbo power than the following will be needed:
Turbo Kit (I like MCX because they are bomb proof and very trouble free, you will get a lot of opinions on this, i personally would only spend my money on MCX, proven !)
Avid Drivers
Clutching (Weights, primary spring, 14.5 rollers, helix, sec spring etc)
Rear suspension (Timber, M-10, Baker, Kmod etc)
Track ? (I ran a stock Maverick to 7000km with boost until it fell apart, Camo Extreme is nice !)
Boost guage
I'm sure there is more.
Don't waste too much money on aftermarket stuff. Half of it i bought and got rid of it later on. It just doesn't hold up like the factory stuff. Just put a turbo on it and ride. Tons of miles with very little trouble. Remember take everything you hear with a grain of salt especially what you read on this forum. Some aftermarket guys will tell you their little magic box will make your sled run way better, they are full of bunk LOL !