Ah ha... yes.. that is a bit tricky. You either need to know the username and start typing that in (e.g. PeterF), and once you have typed the third letter a dropdown box will appear trying to start matching usernames. If you see the one you want, you can select it, or keep typing more letters to refine the list. Or you can co to a forum members profile page (click on their username in on a forum post) and choose the 'send a private message' option on the left sidebar.
However, posting these sorts of questions is exactly what the forum is for, so you have chosen the best place to ask your questions

Firstly, welcome!

So, yes, my experience has been, and I think most of the posts on the forum will back me up on this, is that any of the cheap 3.3v/3V3 CH340/CH340G usb to serial adapters will do the job (as long as they're not faulty, that is!!). If you just want to follow the
easy update tutorial, you make make your life even easier and just get on of the ones linked in the tutorial...
Seller 1 ...
Seller 2 ... and
if you want two (as more is always better).
For the weather shield, it does indeed appear that it refers to the Oak itself... as these are written by both Erik and forum members, there is bound to be some minor inconsistencies in how things are described. I'll edit that so it doesn't confuse!

For the Robot Car kit, there is a tutorial, but it is under the heading of
Robot Kit, so needs to have some mention of the car! Be warned this tutorial is actually for the Digispark, so there will be references to the digispark instead of the Oak, and although I don't have one, I suspect there will be some minor differences regarding connection, etc.
You're welcome. We're all here to have fun and share what knowledge we have, and I hope to see you on the forum in the future