I bought my 18" WSM in 2009.
It was well used when I bought it, and it still has the original door and original latch.
I've never even remotely felt the need for the CB door or a different latch.
YMMV
I'd get the latch. I stripped the grommet out of my weber handle pretty quickly after getting the cajun bandit door. Originally the doors didn't come with the latch. When installing the latch you had to drill out the hole on the cajun bandit door which is a pain. You can do it with a stepped drill bit but I put too much pressure and flattened out my door slightly and had to rebend it. The doors may come drilled for the latch but they might not. I'd save some time and just get the latch.
You can also do like Bob says and just use the original door. It's not worth it to get all OCD about a little smoke leaking here or there. I did put on the door, the latch and the gaskets (because I id get all OCD) but now my gasket is starting to leak after a few years on the top section and guess what? I don't care. I find that once you get it all sealed up you are going to lose some of the higher temp range. Then you are going to be thinking about putting in a second vent in the lid.