On my long range setups I typically have a scope with an objective in the 44mm neighborhood, my favorite being the Primary Arms 4-14x44mm FFP scope. It's feature rich for a $230 scope, but may be more than you need out to 300m. Still, it's not a bad choice. Any of the SWFA classic of the Bushnell Elite 3200 would be a good choice for a fixed scope of the tactical variety.

Just about all my scopes are in low profile Leapers Max Strength rings. Any rings with a saddle height of 10mm would do the trick for an objective in the mid-40s.

I like EGW rails, but mainly because I go for 20 MOA rails. For a .308 at 300 meters, a sloped base isn't necessary, which opens up a number of other options. Weaver makes a nice and inexpensive single-piece rail. There are also a lot of good things posted about DNZ mounts.