ERRATA:
1. So, apparently, some players replaying missions for the 300th time expect you to buy Two-Way Teleporters so that they can do Single Player MvM Theatre more easily. Fortunately, this is not a normal condition in MvM, and you'll only have to deal with it if you play Mann Up and get stuck on a server with loot farmers while playing certain poorly-balanced official missions from 2013 and 2012.
2. Not so much a correction as a clarification: Dispenser Range is more helpful to certain classes (particularly The Heavy) than others, so there are some team balances where Dispenser Range isn't nearly as good as it "usually" is (e.g. 2 Engies, 1 Demo, 1 Medic, 1 Scout, 1 Spy).
3. Sentry Busters will continue to spawn at a set rate until they succeed in destroying their target. If the Busters are annoying you, consider allowing one to destroy your Sentry at an opportune moment and building a new Sentry. This will reset the trigger that causes Sentry Busters to spawn.