What you are wanting to do is going to cost alot of money. First we need to kinda get an idea of budget your looking to stay in. Ru looking to do this really soon and buy all the stuff really fast or do it over a long period of time? The place i would suggest starting is in the suspension area. The 400ex stock suspension has alot of room for improvements. If you trail riding and racing which is exactly what i do with my 400ex you can look at my signature as a guide. I did my build on the cheap over a few years. I did alot of research and purchased with 2 things in mind. Wanted strong well built stuff that can hold up to anything i throw at it and at a lower price point. Honestly alba is where i saved alot and there product has been great for me. The 416 kit has been very reliable. Again do your homework and do all the supporting mods to make it all last.
To many mods to list in signature(ran out of space lol)
Check out garage for full list