Hy Pratyay Bose, it depends on what kind of hotels, restaurants you'll be looking for. but for a good trip it would cost around 12k-15k for 4 persons for 3N/4D
Munsyari lies in the Pithoragarh region. Road are comparatively finer in that region. The rocks are stable and they are not as shaky as ones in Rudraprayag Badrinath region. If you're going in January, you might have to face a lot of snow and icey roads which may not be amazing for a motorcycle. But fortunately that'll be limited and you'll find the roads easily rideable.
You can visit cave temple of Patal Bhuvneshwar made of line rock I guess. You need to be physically fit to descend into the cave hole to enter the underground cave temple. You can also plan for Mukteshwar apart from Binsar. Binsar may not be intriguing but Mukteshwar is a good place to stay.
All in all Munsyari is safe ofcourse. You can make the bike ride quite comfortably.
There will be a checkpoint before entering Goa. you have to stop there as it is compulsory and Goa police will stop your vehicle for checking papers , you have to pay tax over there. and make sure the owner of the vehicle is with you or they will charge you fine.
Hey BULBUL On The Wing, I suggest you to buy online well in advance, depends on the availability some time its difficult to buy the tickets after reaching there. you can visit www.russianrailways.com for booking tickets. warm wishes for your journey.