If you want to race enduros, then you need a street-legal bike. In some cases, just the front/rear lights are needed... check the local rules and also, check with the officials running the enduro.
For hare-scrambles/cross-country racing, a lighter bike is preferred. Check the rules for engine sizes (or wheel sizes) per class.
Electric start is generally a very good to have in the woods, although it adds weight. As you dump the bike or stall it in a screwy situation, with precarious balance, just having a button to push is awesome.
The alternative to having the electric start is spending the time and effort to get yourself and the bike into a more stable position and then kick it... it is pretty common to see lighter bikes raced by the more experienced racers, who don't get into screwy situations often, but you can see the trade-off.
You should look for quieter bikes. Noise doesn't make them faster: the rider does. In fact there are noise limits at most races now.
Too much power? A big 450 for instance, puts out twice the power for the same throttle opening. This causes jerkiness at low speeds and a bike that runs away from you every time you twist the throttle, not to mention that it is a heavier bike to boot. Both very annoying if you just follow the kids in the woods and run the occasional hare scramble...
On the other hand, that power may come in handy in an open loop, where you need lots of speed and less maneuverability.
New to the sport? Get a smaller, lighter, older bike for a season or two. You won't loose much when you sell it and you get enough time to meet and talk to more people, try more machines and figure out the right one.
It's a matter of preference. Two strokes have a different power delivery, making more power at high revs and the motors are usually much cheaper and easier to maintain. They are also lighter for the same power...
Four strokes have a smoother power delivery, have more torque at lower revs but have more weight and are more complicated engines. Recreational four strokes last forever, but race engines may need pretty expensive maintenance (1,000$ for new piston/rings not uncommon vs 200$ do-it-yourself kit for a 2 stroke).
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