Ive read the 45+ page thread on peoples Aero builds and I'm more confused now than I was before.
Right now I have a standard carbine extension on it.
I see some that swear by the Slash heavy buffer and spring. 115.00 plus shipping.
I see that Magpul now sells a longer extension tube. 70.00 plus shipping.
If I get a long extension can i run a standard buffer and spring?
Is the Slash is a buy once cry once deal.
I don't want to stock pile a bunch of 308 parts, they tend to grow into rifles...
I already have two complete lowers so clearly I have a problem.  
Thanks in advance.
Oh and no suppressors here.

No need to spend that kind of money.

I use the following setup on my home made budget Aero 308 rifle and Im almost 300 rounds in without any malfunction. Its a 18" barrel with rifle length gas tube, with probably 90% Aero parts.

Used milspec buffer tube - $30

308 Heavy Buffer - $32
Heavy Buffer
308 Buffer Spring - $6
308 Buffer Spring

Yea, I'm shocked at the prices on these things.
Brownells is oos on the buffer.
Do you happen to know how much the buffer weights ?

I would typically say adjustable gas block but since you indicated no suppressor, I say sprinco green and at least the 3.8oz buffer depending on barrel length and gas port location

I would buy a SLR 7 gas block before a slash buffer which may or may not work the first try.

A baby govnah might be a worthwhile option too

KAK Industry 5.5oz LR-308 carbine buffer  ($55.00) in my Aero M5 308.  with Slash's 308 carbine spring ($15.00).

i also have Slash's Heavy Buffer CAR-10 buffer, bought before KAK came out with theirs.  both work fine, but i do use Slash's 308 carbine-specific spring with either of them.

the Slash buffer is indeed bee-you-ti-ful.  but i don't think the BCG cares.

The barrel I have is  18" Faxon heavy fluted.
I looked up the DPMS buffer, it's 3.8 oz.
The gas block looks to be oos as well.

What springs will work with this buffer.

when i bought the Slash Heavy Buffer, the KAK buffer wasn't available yet, so the Slash buffer with the Slash specified spring served to confirm that this M5 308 combo worked fine, which it did, worked perfect.  since the KAK buffer weighed the same, i bought that, measured it, same, and then installed it.  also works fine.

so anyways, i'm squared away on 308 carbine buffers for my next M5 lower build-up.

i would only use the Slash carbine 308 spring, otherwise it's a whole 'nother trial and error exercise (time & money), for me, at least.

YMMV.

On the other half of your question if you are currently running an AR15 buffer tube with the short DPMS 308 carbine buffer (CAR-10, 2.5"), than yes if you change to the 3/4" longer Magpul / VltorA5 / AR10 buffertube you can then use the standard AR15 carbine length (3.25") buffer.  On my 308 platform I went from the short set up to the longer A5 tube with a Tubb 308 flatwire and a standard AR15 carbine h3 buffer(5.4oz).  I love this setup but if go to this you will need a long stock to collapse all the way otherwise you will have it caught in between the first and second position.  The long stocks are Magpul SL, ACS, STR, UBR G2 and the Vltor EMOD.  I may have missed a few but thats all I can think of or you can get the Vltor A5 6 position tube which allow an M4 length stock to collapse 1-6, heres a video on it  click

Sprinco green