IRT RRA: There are tons and tons of RRA threads on THR. Go to "Search" and select "Advance Search", and run a search under "RRA" with the "Titles Only" button pushed, and you'll get dozens of threads.
Depending on your location, $800 sounds a little high, unless the Red Dot is a quality brand (cheap red dots aren't worth much).
You can buy a complete RRA lower (fixed or adj stock) as low as $229. You can buy complete RRA uppers as low as $450. That's $679, add $20 for shipping and $25 for FFL fees, and remember there's NO SALES TAX. Up to $725 now. Leaving you $75 (and another $50-70 depending on your state's sales tax) with which to buy whatever carry-handle, BUIS, or optic you choose.
Just tossing out another option.