The 20" length is fine as long as the twist rate is adequate to stabilize the slug ... which the barrell maker should have taken care of in the barrel design. As a matter of choice, my center fire rivle that I hunt white tail deer with has a 20" barrel .... I find this rather compact size ideal for easy handling.

As for a scope, in your $200 price range or $50 or so above, there will be several decent options. I would suggest a heavy plex type reticle. ... German #4 or German #8 for example. This wider reticle is excellent for alignment in low light or to get on target quickly. But this is just my personal preference.