The question I would ask first is what are you going to do with it? Next I would ask what is your experiance level. And last I would ask what do you hope to accomplish with it.
I'm far from an expert but here are my opions for just the average rider 180lb + not hard core racing or dragging just good solid fun. When all is said and done to your motor I would put it on a dyno wiht an a/f meter to rejet/tune it to what you have done to the engine (not for how much hp you have, its nice to know but not the main reason to dyno it)
for sand
either an HRC kit or to maximize power and your going to want to do it later anyways you can skip the hrc kit and go with...
Depending on the area either a DASA (no sound restrictions) or a VENOM (for area that has strict sound restrictions) Full Exhaust system 500 bucks
HCS2 around 100 bucks
A new set of haulers size and # of paddles depends on where you are 400ish bucks
remove air box lid free
cut green wire on harness (find the article here to do so I'm not your secretary

) free
outerwears prefilter either the one that covers the lid (the one I use) or the one that goes on your filter. the K&N is nice but gives you little bang for the buck 50 bucks
Then when you can
Port and Polish
Extended swing arm +2 general
Suspension work to suite
for MX and XC
I don't realy know enough about it to make solid recomendations but minor tweaks like leaving the air box on for xc and more suspension work than motor work for both of these since mx and xc are more technical and less pure hp related sports.