I tried searching a bit and the internet sucks for finding .308 stuff.

I bought a stripped m5 lower on a whim and now it's sitting in my safe collecting dust.

Want to finish it slowly over the summer as a birthday gift to myself but I need some advice because .308 stuff is weird.

1. What stock/buffers/etc work? Can I just use 5.56 stuff here?

2. What uppers will fit? I know an aero is the obvious option, but if I see a deal on something else I may jump if it will work.

3. I've read about varying heights on the uppers affecting what rails I can use so that opens a new can of worms on what rails work.

4. Are there any other things I need to know?

Standard extension works but you need a .308 buffer/spring. ETA- might as well order Aero's  M5 kit.

Stay with Aero for the upper...period.

DMPS High is what you will need.

Yes, stay with Aero for the FCG and BCG. From there, I recommend a barrel from Ballistic Advantage and a rail and gas block from SLR.

ETA- this is just one of my M5 builds and it shoots like this all the time...

here is a site that will help you with your build I used it when I built my Aero 308

https://308ar.com/

and I bought some parts here

http://www.schuylerarmsco.com/

and here (their 308 BCG works great)

http://www.tomstactical.com/

here is what it looks like, spent around $1300.00 building it (not counting the scope) but it now wears a meopta meostar R1 4-16 mil dot.

Any dpms high configuration will work

Recommend the adk arms bcg.  They are cheaper than aero but great.  Have 2, sveral thousand rounds through them no issues

I used a aero upper and a faxon barrel on my build. For a BCG I used a Toolcraft and it runs great.

Aero upper, lower and bcg. Ballistic Advantage  18" barrel. JT rail in dpms high. Runs great with no stoppage for 300 rnds so far.

Will a dpms lpk fit?  

That's step one of the build.

And will the geissele trigger from an ar15 work?

The take down and pivot pins are not long enough in the DPMS kit. Neither is the mag catch. Everything else is standard.

I suggest getting a lower kit minus trigger group from Aero. Any AR15 trigger *should* work.

I would suggest a better bcg, one from JP, Lantac maybe, or something better than Aero. I would trust the DPMS bcg before aero. There seems to be significant amount of issues with these rebranded bcgs Aero and many others use. Ranging from bolt issues, pressure and gas relief issues, the rails being too long causing damage to the receiver extension boss, etc etc. QC is almost nonexistent with these newer 308 offerings. They realize the large majority of people don't run their rifles much or hard, so they are lax on the QC. They are betting on the odds you will not run into issues before your warranty is up. Or ever notice it at all and just accept the platform as being problematic.

Like another poster said, "no problems within 300rds". That's nowhere near enough to even begin to judge how well it will perform.

You gotta determine what you're really going to do with the rifle. Since my babies have been born, I'm only averaging 1000rds a year the past 2 years. If you don't plan on getting anywhere near that, go budget for everything. But, if you plan on really counting on your rifle, depending on it to work, get ready to put money aside for testing, buying multiples of certain parts, more testing, etc etc.

First thing first, determine your plan for the rifle and go from there. Study the FAQ here, and other sites as well if you like. The one here is the most extensive AND accurate.

I shoot my rifles about 5-6 times a year and pistols really frequently. I have my first kid on the way so we'll see how that changes in a few months.