In reverse order, there is no rear sight provided with the kit. You could go with any of the carry handle/mounts, a folding flip sight, or any of the stand alone rear sights available onthe market.
The first question is harder, because of your intended use for the varmint rifle...you want to cover a lot of territory with it. 1:8 twist would be best for the heavier/longer bullets, approx. 62 gr plus, but some shooters are getting decent results with slightly lighter rounds as well. The 1:12 would be better for the lighter/shorter bullets...pretty much 55gr or less.
Thanks for asking.