As long as all charity money goes to the charity and not toward your vacation, that's fine. A LEGIT charity raiser will have all funds sent directly to the charity, not to their PO Box, home address, or Paypal account. We've had way too many people use the money for pay for their vacation (aka expenses) and then donate what's left over (if any) to the charity, then claim it was a charity ride. Asking for gear is fine, but I also believe that you should donate that gear to someone else when you are done your ride to spread the charity.
Now as far as your ride, the idea of doing 2-3 day tours to adjust yourselves and your gear is by far the best advice. Also be a good idea to do an over-nighter on a night calling for harsh conditions and/or rain. By doing these short tours, you'll be able to tweak and adjust what gear and equipment fits you best.
As far as getting a "touring bike" goes, if what you are riding now is comfortable day in and day out, why spend money you don't need to? You can tour on almost anything. I don't use a touring bike at all, never have bought one, I use a standard Walmart quality mountain bike.
Anyhow, if you and your brother go through with this, I wish you the very best, and you'll have a lifetime of memories.