Nice build, it depends on what your purpose for the rifle is. For a good red dot I would go with the Trijicon MRO. For a good medium to long range fixed scope I would go with a Primary Arms 5X Prism with the ACSS reticle (sort of a budget ACOG). For a variable scope I would look at the Primary Arms 1-6X ACSS. All of those would be half your build price or less. If you want a more premium optic then a true Trijicon ACOG, but it would be about the price of your build and up.