I'm building an upper (almost from scratch).  I am ordering a barrel and an upper and a FF handguard (possibly a FIRSH).  

My question is whether I need to get a delta ring, weld spring and snap ring if I am installing a 2 piece ff handguard?

Nope.  I put a FIRSH on mine, and it came with the barrel nut and the tube.  All you need other than that is the barrel, FSB, gas tube, and upper.  If you want a FF handguard, get it and install it now, while you are building, cause if you get all the other parts (delta ring, weld spring, snap ring, handguards) now and decide to get a FF handguard later, you will have all those parts laying around.  BTW, I really like the FIRSH, although it has grooved-in-the-middle "Picatinney" rails rather than mil-spec 1913 solid Picatinney rails.  I haven't found it to be a problem, the handguard is strong (dunno about putting optics on it, verdict is still out), and the price is great.  Mark Buddam at Triple Break Products has the FIRSH.  He is a good guy and will set you up with what you need.  

Ghost

I have a question along the same lines.....is a barrel extention necessary when ordering an aftermarket barrel such as a bushy barrel?

I agree!

All of our barrels RRA come with the barrel extension installed, I believe this is the case with most manufacturers
Robert
www.rbprecision.com

Nope, it would be very hard to find one without it.  None of the name brabd barrels will give you any problems.

I agree...

If you buy the barrel from an AR manufacturer or parts dealer, it will almost always come with the barrel extension attached and headspaced.

However, if you actually buy from a barrel manufacturer (like to get a super duper accurate barrel) you MAY have to order the extension separately.


BTW, thanks for the answers guys.