Questions About single speed tranny...

i have a few questions about the single speed tranny? Does this kit eliminate the first gear or second within the tranny ?

Would it be worth it to buy the traxxas 7075 t6 input shaft/drive gear, idler gear shafts, and output idler shaft ? as well as the hardend steel idler gears, and if so how much modification is required ?

I'm looking to make my tranny/driveline bullet proof and the tranny as light as possible