Buy the kit you want and run the 300 this winter. If it sucks you are only out the cost of a fit kit and still in the same situation next summer for finding the "right" bike. The only way this doesn't work out for you financially is if you plan to dump a boat load of cash into your 300 for motor work and fully deck it out.
It's not like you are going to be sitting at the bottom of the hill because of your choice to feel it out. Everyone spends so much time spinning their wheels trying to find the "perfect" bike their first season out. I've got a secret to tell you... there is no "perfect" bike out there.... just a bike you like more than others.
Couldn't agree more with this comment. I have ridden 3 different setups on 300's now, 2 different CR500 setups, and I've been on my 2008 450 since 2008. All in the snow. I think I could fairly say I have enough experience to adequately respond to your question/dilemma. Sorry to say but there is no one-bike-does-it-all and everyone should own THIS seup answer. Every setup has its pros and cons. Just run the 300. When you run into other guys, ride their setups and see what you think.
Here's my own OPINION on the setups mentioned. You know how opinions are... Take it for what it's worth
The 300 goes fine when in the power band, but when not it just doesn't go. It vibrates too. But it is a light setup and great summer bike.
The 450 has awesome tractor power and can be lugged around like crazy, but is heavier and even after 15 different ways of routing my vent tubes, I still get water in the oil every know and then. The 450 is so smooth running that vibes are not noticed or ever an issue. It's carb doesn't freeze either, at least mine doesn't.
The CR500's have kick butt power and are light, but vibrations are an issue. Not a big issue, but an issue none the less. Carb heaters are required on the two strokes.
I've come to the conclusion that there is no perfect year around bike. There isn't even a perfect dirt bike for that matter. Where one excels in the woods another excels in the desert. Same for snow. Buy one for each riding condition you enjoy is my advice. I'm still looking for money to follow my advice!
Just get a kit and get in the snow.