Your goals are somewhat contradictory. Buying a fully built bike is almost always the best bang for the buck, but its difficult to find complete bikes with SRAM Red which is by far the lightest groupset. A complete bike will likely have better resale than a built bike, simply because buyers can better gauge the initial price. On a lightweight build, you'll likely use higher grade components but a prospective buyer will assume generic stuff. Going into this, you should be aware that high-end bikes don't hold value particularly well. Components trickle down, frames improve, etc. A few years ago, <1,000g was a top-tier carbon frame, now many entry level carbon frames are 1.2kg.
Electronic groupsets are expensive and slightly heavier than their mechanical equivalent, so its a double hit to your goal of light and not expensive.