Apart from the REO and the RM2, I would definitely order a set of replacement screws for the RM2 (you will lose at least one of them within the first week I almost guarantee it)
The O rings are to allow you to position the airhole on the RM2 correctly. I believe that when they are drilled, the same sized O ring that is supplied with them is used, so it should line up right out of the box. I know mine did. I think they are 25 cents, so if you want, get one of each size. You can always experiment with them.
I don't know about buying a new spring, I don't have the expectation that my spring will fail anytime soon so all I got was the extra screws for the RM2, some extra
juice bottles and a brass cover for the button.
You might want to get a couple of extra bottles for
juice. They are pretty hardy but I like to switch out my juice a few times a day, so having extras helps. If by chance you have any of the small bottles of juice from Fancee Juice, they are the same bottles.
EDIT to add that if you forget something they are usually pretty good about maintaining inventory of the little extra stuff you may need, and shipping is about $2.50 so it won't be the end of the world if you need to order something later down the line.