You need a chain tool for every chain, OEM or aftermarket, unless you plan on removing the rear swingarm.
The OEM chain is the DID 525V[8]. DID has different quality chains for the 525 size, see
their spec sheet. As far as I can tell, the DID 525VM2 is widely recommended as a replacement for the OEM chain.
When I was looking for it, the 525VM2 was only available in gold, which felt wrong for a dual sport bike... (not sure why people are crazy about gold chains).
So I went for the DID 525ZVMX which is overkill for the bike but I was very satisfied with it. Unfortunately, the lifetime of the bike was shorter than that of the chain, but for as long as I was using it, it was very quiet and I never had to adjust the chain (no stretching whatsoever).
With the wide variation of mileage that people report on their chains, I think that riding style and maintenance habits are much more significant than X vs. O etc. Wouldn't worry too much about it.
It is recommended to always use new sprockets with a new chain because their wear in together. Using the old sprockets might shorten the lifespan of your new chain. I always use OEM sprockets, particularly to have the rubber damping in the front sprocket.