I'm converting a 250R and the stock clip chain and rusty sprockets need to go, I've been researching and it sounds like the 428 setup is a mythical setup that may ...or may not.. be worth the time, effort and cash.

Stock is 14/45, and it sounds like most racers are running that or 15/45 for a little better top end. I'm guessing I should stick with the stock sprockets with a 520 chain?

Keeping in mind I'll be racing on a track that's 2.25 miles, fairly technical and has a straight away that will allow me to get up to max speed.

What setup would you suggest and why? (front/rear sprocket, chain + links in chain)
Who would you recommend purchasing from online?

I don't recommend the 428 chain conversion for the high cost and low gains, especially if you're not in a VERY tight battle for the lead in a race series. In the AFM series, at least two of the top three guys are running 520 chains.

If you really want to do it it should "free-up" about 0.5 hp and reduce some weight.

Down here most people are running stock gearing or very close to it. Usually varying only in going up or down one tooth in the rear, depending on the track.