OK guys, I did this trip myself last year, so here goes.
1st, find a bike, try ...hamrobazar.com :: Nepal's Online Marketplace
it's like a Nepali ebay, with loads of bikes for sale, and you can check it out from home !
2nd, before you can register the bike in Nepal, you will need a 'Letter of no objection' from your Embassy, easy enough, just takes time and an extortionate 'processing fee'.
3rd, CDP, you cannot get your CDP untill you have your bike, this can cause problems, as whilst theoretically, the RAC of Nepal can issue them, in practice they don't, so most people get them from their home country, I got mine in UK ( I'm a Brit ).I would suggest contacting the issuing org. in your home country first, to inform then of your plans and confirm that they are agreeable.
4th, Visas, unless you have residence in Nepal or India, you will have to apply for your Pak visa in your country of residence. If you are entering Nepal from India, then make sure your Indian visa is multiple entry, as you will be re-entering on your return.
5th, depends which route your taking, as both China and Iran have their own unique challenges !
Good luck, UB.