I have not use a BCM 20", because I prefer 16" or less... but a BCM wityh a midwest rail would be pretty perfect.
When I build my next AR, it will be all BCM and I plan to use either a Midwest or Samson rail. I have several Samson's and they are great. Price is similar to Midwest, so it just depends on what they have available at the time.
Looking on the BCM website, it says the BCM 20" does not include bolt carrier group or charging handle. Looks like $150 more for the total price.