Standard carbine buffer/spring? Or something else? I'm wondering with a lightweight BCG and adjustable gas block, if I should go with something like the JP low mass buffer.

rifle or carbine stock?  

The lighter the better.  A gutted carbine buffer works well, or you can use something like the Taccom ultra light buffers.  JP stuff is good to go as well.  You can also use a reduced power spring if you want.

JP Silent Captured Spring. Will work with carbine or rifle length tubes. Comes with a spacer to use with rifle length.

Related gun building thread

ALL BELOW ASSUMES ADJUSTABLE GAS BLOCK

Don't go below 10 oz of reciprocating mass.  Aluminum bolt carriers are NOT worth the hassle or money.  The Titanium one is nice, but not likely worth the money.  LMOS or any other lightweight steel bolt carrier is good with Taccom or gutted carbine buffer.  You can also use the SCS with the weights removed.

The WEAKEST spring possible is best with adjustable gas.  The best weak spring is the 80% spring on the JP SCS with spring kit hands down.  Next is the 10% reduced springs.  Taran probably has the best 10% reduced power just because of the quality material and coating.  Gunshow Car springs are very similar.  You DONT want Mil Spec buffer springs in 3 gun or game rifles.

My last build.

http://www.taccom3g.com/223-adjustable-recoil-system.html

My next build ....

See above.......