Welcome and what a fantastic opportunity. It would be great to see some pictures, maybe even a photo diary from purchase-to modifying- to journey. We'd all be busting to join you by proxy.
As for mods (O2 device), Kev sells them on ebay.
https://m.ebay.co.uk/itm/Yamaha-XT66...UAAOSwHnFVw0N-
Your request for overlanding info will keep your thread busy for a while, I guess.
My humble initial input is to try to take off the rear tyre yourself in the comfort and privacy of your own home. It's a skill that may well be a God send should you be unlucky enough to catch a puncture in one of The 'Stans' where the RAC don't quite get to. The rear is very well known to be a complete pain to get off. This way you'll also realise some of the tools and kit you'll need (small 12v air pump / Rocky Creek, for one thing).......and a 14mm hex bolt to undo the front wheel spindle too.