Your weight questions will be hard to answer b/c an 18.6" bbl isn't all that common in the US....though I'd agree that 18.6" makes a lot more sense in a .308. Just not sure there are a lot of them around here to weigh.
I'm assuming you are talking about the standard length handguard as well. That will also have a slight affect on weight if you go mini vs. stnd. vs. comp length.
The buffer will last 10's of thousands of rounds unless the gun is severely overgassed. It will start to come apart if that happens...deteriorated, chunks missing. Never personally had it happen to one of mine. It's a wear part. Get one. That's probably all you need unless the gun has an overgassing issue.
There is a SPK (Small Part Kit) that includes the buffer that you can buy from RA direct or Navalbeaver here on the site....normal pricing is something like $125-150ish. I'd also recommend NOT ordering the complete M direct and ordering from Navalbeaver or Gramps96Kz here.
There isn't really any maintenance. Clean it if you want. Oil them if you want. Or don't. I don't. They just run.