Do yourself a favor and get your eye behind a Vortex Diamondback 3-9x40 ($200) and a Viper 3-9x40 ($300) before you make a final decision.
Opticsthoughts.com did a review on "inexpensive" 3-9x40 scopes and the Vortex Diamondback came out on top - “Inexpensive Scope” Comparison » OpticsThoughts
If you're looking to shoot long range with an AR platform, you'll definitely want a cantilever style mount to keep all the mounting hardware on the upper receiver and avoid having one ring on the receiver and the other on the handguard. I'm personally not a huge fan of the LaRue mounts because...
Be ethical and keep the 22-250 for varmets like you said. Yes, a 22-250 could kill a deer/antelope but it is not an adequate caliber for 600 yard shots on them. Look into a .243 if you want a more "all purpose" gun.
Viper HS LR all the way. Mine has been flawless and alot of fun to take to the range. Haven't had the opportunity to take an animal with it yet, but I feel pretty confident when that time comes.
Reticle thickness is such a subjective topic and there will never be a reticle that is perfect for everyone. If you have trouble seeing the BDC, then get the Plex reticle. For me I prefer the slightly thiner reticle and have no problems seeing it in low light or on darker backgrounds. So far I'm...