Planning my build now.
Using Aero M5 set. Plan on going with 16" barrel from Mega with a mid length gas system. I will shott supressed a lot, so I want an adjustable gas block.  With the adjustable gas block, is a heave buffer needed, or would it be redundant at that point?  I'll have a rifle length receiver extension and buffer/ spring

It depends on what kind of gas block you get. If it's something like the switch block, you may still need to choose the right buffer for best results. If it's a fully adjustable gas block like the SLR or Syrac, don't worry about buffer weights.

I just tried my first adjustable, got the Syrac. On a 16" with carbine gas I am running about 3/4 closed unsuppressed, so it was pretty over gassed. A heavy buffer would be unnecessary with a fully adjustable block as they are both slowing the bcg.

A heavier buffer will slow the BCG down.  It adds to the weight of moving mass.  Great on single shots and slow firing.  Adj gas block can be tuned to only use the gas needed to move a BCG and lighter buffer.  Great for single shots and slow fire...AND rapid firing.  Great for faster follow up shots.  The buffer is the cheaper route.  If op decides to go with adj gas block I'd recommend the Syrac Ordinance gen 2 gas block.  I love it.  I hear good things about SLR also.

I dont know about buffer being cheaper, as I see the heavy slah is $100. That's what i can get the SYRAC for. So I ordered the SYrac.

thanks for the advice.  I know I'll be back with more questions

I say both. But only because I like knowing I have a quality piece added to the action in slashes products. Cant wait to try an adjustable GB though.