I suggest doing some Hare Scrambles before enduros. They are basicaly the same type of riding except you do laps. You can do 2-3 laps and then quit or you can go for the entire thing. The laps are generaly between 5-12miles depending on the series.
The KDX is an awseome woods bike! The 400cc (exluding CRF's YZF's and WRF's) bikes are a bit heavy for a new rider to be racing (IMO), but they are very reliable (as the KDX is).
First thing to do when you get the bike is too take it apart and greese all the bearings and check the torque on all the bolts! The factory has a tendancy to undertighten and under greese things! Make sure you break it in properly!
You will need to jet the bike ASAP. The factory always sets the bikes up rich for waranty concerns. If you don't know how to jet carbs., then learn. Jetting is a VERY good thing to know! Take the time to read some articles (
www.eric-gorr.com for starters) about the subject.
You will probably want to get a good ride-along tool kit (20-70$) for any trail problems/breakes that may arise (especially on enduro's). There are a few threads on this site that discuss the tools that people carry with them while riding/racing, you may want to check those out.
You will definetly want to take off the stock hand gaurds(KDX) and put on some barkbusters (they have a metal rod so you don't break fingers when you hit trees). The stock bars will also have to go (80-130$ for pro-taper and renthal), but don't worry about that until you tweak the stock bars real good. The front springs will also have to go ASAP (80--90$ new). While your changing the springs you will want to change the preload spacer (1inch 40schedule PVC pipe or cut stock metal spacer) to suit your weight and springs. Set the race sag on both the shock and forks too (FREE)(don't forget, this can make a big difference in your ride). Also a oil adjustment may be required. Definetly get the skid plate too (around 60$?).
As for boots I would recommend getting the tech 8's from alpinestars (280-325$). Yeah, They are the most expensive, but they're also the most comfortable and durable IMHO. Well, worth the investment. For the helmet I would go with shoei or Arei (500+$). These are expensive helmets , but the strength and comfort can't be matched (you can get the discountes upto 250$ for last year models). If you don't want to spend that kind of money for a helmet I would consider the Thor helmets (SVR2 or something like that) as an option. No matter what I would get a helmet that is on the high-end (I know my head is worth the extra cash!) A good riding outfit would also be good. You can find pants, shirt and glove combo's for 90-150 bucks(last year discounts). Thor phase 4 or oneal gear wouldn't be a bad way to go. I have a phase 4 outfit and it has held up well, I would buy the new core 5 or AC 5 if I could afford it! If I was going to get any braces I would go with EVS, though they can be expensive. For the goggles I would go with spy or pro grip (I use pro grip right now) light sensitive goggles. You will want to get a good gear bag too. You COULD buy a big school bag from walmart but I would go with one designed specificaly for the sport (70-150$ depending on size and brand)... it just makes life easier! I use a no fear bag, but I am not real impressed with the storage in this particular model. They do make great bags though. I've had it for more than a year and a half and it looks brand new. I just wish I had gone a little bigger (and with wheels)!
You should probably look into getting all the protection you can on your bike (IE pipe gaurd (40), caliper gaurds(20$), brake disc gaurds(25$) ect.) You will most definetly break stuff anyways, but the more protection the better!
My estimate is about 5-6000 to start off with a KDX and more for the more expensive bikes. It is expensive to start this sport, especially racing, but most of these things last a long time so the biggest price tag is getting started.
It will probably take AT LEAST a year to get your bike set-up just the way you want it (based on money and testing time), but there is nothing sweeter than having your bike set-up perfect for the way YOU ride (power and egronomics).
Good Luck,
Tom
edit**
A uni-air filter or twin air filter is a must. do the airbox mod too (you can remove the lid or drill it to get more flow and add to low-end response/power)
All the prices I've listed are average list prices some can and will be cheaper or more exphensive.