I like the site D&D Beyond and may well end up building more stuff there in the future. But for now I don't think I am going to be buying the resources there. Each book seems to be about $30 - at least the Eberron book in which Tann's warforged character can be found is. While that isn't a ton of money, it also isn't nothing - especially if I end up needing to recreate an entire collection I already have in paper. So what does this mean? It means not everything is available to everyone on that site. However, I don't think there is much reason not to allow people to use those materials if they want to do so. The options as I see them are:
1. Just use the basic stuff from the PH and it seems to let you build what you want from that.
2. If you want to use additional materials you can still do it on D&D Beyond if you have access to another campaign in which to build the character - as Tann did. If you want your character on this site, perhaps Tann would be so good as to input the info for you if you need access to additional materials.
3. The traditional Red Dragon Inn way still works. You can always make up a character and send it to me - I'll post it on the first page of the Q&A section for everyone to see. It would mean that passibly not everyone's character is in the same spot, but it works. Most of the books can be found as a pdf online if you want to use them.
4. I may look into buying just a few classes or races if that is needed as it seems that might be doable at a lower price, but I don't think it is a priority.
***I want to stress that I don't think any of the things in the other books are better then the basics. You know how the game works, if you want an advantage in one area, you need to give up something elsewhere. If something is unbalanced it won't happen. So I don't think anyone is missing anything.
Tann's character and Mischief's character can be friends of a sort if it helps Tann assist Mischief to learn things. Since the group will know each other before hand anyway (at least mostly) I don't see this as a problem. I also know that Tann has been quite clear in his games that in-party fighting is not acceptable, so I don't think this will be done in a way that breaks the party up in any way. In fact, lets just say it won't be.