An Echo dot is just a small Echo. Major difference is sound quality. If you want to listen to music, get the echo. If you just want to control devices, get the dot. You can also hook a dot up to an external speaker, either via wire or bluetooth, so if you already have one and can use it with the dot, then you don't need the Echo. That said, I currently have a dot with a bluetooth connection to an external speaker, but you have to physically turn on the speaker and my wife never does that, and I can't stand the sound quality from the dot, so I just ordered a new Echo to replace the dot.
As for the Echo Plus, that adds a smart hub. But depending on what light and door controls you buy, you may not need it. Some smart home devices require a separate hub, hence the Plus. But I have Wemo light switches and outlets that work with my Echos and Echo dots without the need for an extra hub. I just set them up using my phone, and they work great. I believe there are door looks that will do this also, just make sure they say "work with Alexa" and don't require an extra hub.